So, I will continue with my lecture on cancer in the immune system. We have already discussed
a few aspects of antigens and the immune response to cancer cells, but I will go on now, more
towards the management of cancer. Now, just to recapitulate your memory, let me say a
few words with regard to the immune system and why do we study cancer under deficiencies
of the immune system. This is based on the fact that it is only when there is, not only,
mostly when there is an immuno compromised condition, cancer cells tends to proliferate
or establish themselves. So, this is supported by or this supports
the immuno surveillance theory. The immuno surveillance theory says that our cells are
constantly undergoing mutations. They could be point mutations or more deleterious mutations.
There are always changes induced by mutations in cells and these changes, especially if
they are on the cell surface can be detected by the immune system very efficiently and
therefore, they can be deleted because of the change on the cell surface, these cells
can be recognized as non-self. Also, changes in the on the cell surface can also be recognized
by NK cells, which you probably have already studied by now.
So these there is an effective immune response against these changed cells, cancer cells.
Now, the same thing also is true of virally infected cells. Virally infected cells alter
on the cell surface because of the expression of some viral antigens. So, these are also
thereby I mean you can have an analogous situation with cancer cells as we have discussed in
the previous lecture that carcinogens as well as say, UV radiation induces changes on the
cell surface. Now, this can happen, you know that mutations
accumulate throughout our lives. So, this of course, even we know that cancer is not
a disease which occurs because of 1 mutation, but because of accumulated mutations. So,
one can ask a question, why these accumulated mutations which would reflect in the change
and drastic differences in the cells which have undergone this change, as apart from
normal cells which would not be recognized as non-self, why then there would be no deletion
of these cells. So, it is believed under the immuno surveillance
theory that individuals, when they are in an immuno compromised conditions, these mutated
cells are not recognized as non-self or there is a weak immune response. Therefore, they
go undetected and because cancer cells have a better opportunity for cell division and
growth, they soon form a mass of cells and once this mass of cells keeps growing, then
of course, cancer is established. Now, is there any evidence for what I have
said just now? Yes, there are 2 examples that I can quote. Conditions of old age - I have
discussed this long, you know several lectures ago that efficiency of the immune system starts
to decrease with age. Now, it has been seen that not all, but several cancers appear beyond
a particular age. Therefore, we know the correlation between old age, decrease in efficiency of
the immune system, thereby mutations which would lead to cancer, such cells cannot be
recognized or even if they are recognized as non-self, the immune response that is generated
is not adequate to delete and therefore, yes, cancer you know takes hold.
The second example, I can give which probably all of you all would know is infection with
HIV. Human immuno deficiency virus, we know is the AIDS virus. Now, which are the cells
that the virus infects? It is the CD4 cells. CD4 cells, as you know by now are the central
regulators. They regulate the B cell proliferation and differentiation as well as the cytotoxic
T cell proliferation, which would mean that now with this helper cells or the CD4 cells,
now coming down with the infection with HIV, which would now decrease the numbers as well
as efficiency, it has been seen that individuals were infected, you know AIDS patients, who
already started showing symptoms of immuno compromised state, now start to appear with
cancers and there are certain cancers say, of the skin which otherwise are not very common.
You know these uncommon cancers start to appear in aids patient. So, here is the evidence
one can give that in fact, decreased immune system or immuno compromised conditions can
allow cancer cells to go undetected and survive.
So, there is any way from the previous lectures there is one can say that there is ample evidence
that tumors express tumor specific or tumor associated antigens. You do remember in alpha
beta protein as tumor associated antigens and there are tumor there are experiments
that have shown that most tumor cells express tumor specific or associated antigens at least
to begin with, you know at least when they are established, may be not in the course
of their development of the disease itself, but initially, yes. So, the question is, in
spite you know though they have antigen expressed tumors, are these adequate to trigger an immune
response. Do we have evidence?
So, the question again I can put it in another way. Is the immune response generated to one's
own cancer cells? Is there an immune response? Is there any evidence? Again, there are very
large number of large number of literature which gives this evidence, but let us talk
about the experimental ones because here we can actually, you know in mice, one can induce
formation of tumor and one can look at when the antibodies or the cytotoxic T cells appear.
And yes, experimentally, it has been shown undoubtedly that there is immune response
generated to one's own cancer; there are circulating antibodies seen to tumor specific antigens.
Now, this of course, is also been seen not only under experimental condition, but also
in human that there are cytotoxic cells specific to tumor cells detected. Now, this again experimentally,
has been proven undoubtedly and in both cases, let me finish the third one - natural killer
cells. Now, natural killer cells are part of the innate immune system to some extent
because they recognize non-self and they evoke an immune response immediately by killing
the cells. Natural killer cells as well as activated
macrophages, they can kill cancer cells and these cells increase in number, when there
is a tumor. So, all these three have been shown not only to get activated, these cells,
you know do and they can recognize specifically cancer cells, not only that, they are able
to now kill cancer cells specifically, cytotoxic T cells as well as NK cells and by direct
cell to cell attack in case of circulating antibodies, antibodies, if complement is provided,
antibodies also can kill. So, we know then that yes, there is establishment
of cancer, which is a non-self-cell. Now, cancers are recognized as non-self by the
immune system and there is a response generated. So, in spite of this, why is the immune system
not able to delete the cancer cells? Why is the cancer allowed to get established and
then progress is a question mark.
So, that is because cancer cells can evade the immune system. Now, is there evidence
to say that? There is evidence. Again, there is huge literature which actually discusses
on the mechanisms by which cancer cells can evade the immune system. Let us go one by
one with these. How have people shown that? Now, the first one which is with respect to
antibodies actually, was quite surprising. Now, what people were doing earlier is they
establish a tumor in an animal model. Now, resect the tumor, take it aside and grow
the cells out in culture. Now, take the serum sample from the same animal as well as the
cytotoxic T cells from the same animal and now, look in vitro outside the animal, whether
antibodies can do anything to the tumor cells. Well, it would be again [here self] and build
the cytotoxic T cells. Are they able to kill the cells? Now, in such a system, almost 100
percent of the time there was killing. However, interestingly, when the tumor is in the mouse
and well, even the serum and antibodies in the mouse, it has been seen that in the mouse
in fact, even though, there is good level of tumor specific antibodies, there is no
inhibition of tumor growth in several types of tumors. not only that So, that means the
antibodies are doing nothing, not only that, it was also seen in several instances that
anti-tumor antibodies actually, enhance tumor growth.
You know how do they do that? Now, they had mice which they have induced tumors in and
there are they are checking for the progression of the tumor and checking simultaneously for
production of antibodies. Now, they found that though there is the production of antibodies
specific to the tumor keeps on increasing, the tumor itself starts growing much more
than there are antibodies. Now, these were called blocking antibodies. Why should antibodies
block? Now, especially when there is in the case of a solid tumor, we have a solid tumor
and where will the antibodies bind? They would tend to bind to the cells, which is in the
outside of the tumor. First, tumor penetrance may not be very good with respect to antibodies;
antibodies are large molecule. So, let us say that they bind to the outer
surface. Just binding of the antibody to the cells would not do anything, especially in
case of a solid tumor. If they are cells, which are in case of leukemias, it might be
a different thing, but let us say, in case of solid tumor, the antibodies bind. Now,
for the antibody mediated lysis to take place, there should be complement activation. Now,
complement should be there. Complement components are present in circulation in the blood, but
complement may not necessarily be adequately present around the tumor for the triggering
of the complement cascade to take place. So, therefore, now, anti-tumor antibodies
may enhance tumor growth because the antibodies block. How? Apparently, it is believed that
the blocking in fact, the antigen is being to some extent protected from cytotoxic T
cells. So, the blocking antibodies are blocking the cytotoxic T cells from getting to the
tumor cells. Now, is there again evidence, further evidence that says anything about
these blocking antibodies? Yes of course, experimentalists have gone on further to show
the mechanism. They have taken serum from animals with progressive
tumor growth. Now, they have taken this serum out and they have done in vitro CTL assay.
That means target cells being killed by cytotoxic T cells, in the presence of serum from animals with progressive
tumor growth or serum from regressing tumor. Interestingly, they found that serum from
animals with progressive tumor growth always almost always inhibited cell mediated lysis.
So, obviously, again these could be blocking antibodies.
So, we know now, the different ways so far, how tumor cells are able to evade the immune
system. Remember, there are antigens, but there are blocking antibodies, which can inhibit
the generation of an effective immune response. There is another mechanism. Tumor cells express
low levels of MHC-1 molecules. By now, you would have studied the way CD8 cells or the
cytotoxic T cells can bring about target cell killing. In fact, T cells recognize peptides
from antigen or antigen peptides in the context of class 1 MHC or class 2 MHC molecules.
All nucleated cells have MHC class 1 molecules. Now, in case of cancers, if the cancer cell
has to be recognized as non-self, then there should be these cancer specific antigens which
are loaded onto class 1 molecules, which can be recognized by cytotoxic T cells for them
to effectively kill. So, what happens to MHC-1 molecules on the tumor cells? It has been
reported that tumor cells start down regulating their MHC molecules. So, what is shown in
this picture is that original tumor cell here has the same number of MHC molecules which
a normal cell would have. Now of course, tumor cell is usually larger.
So, by that logic, one would have more MHC molecules, class 1 molecules I am talking
about, per cell. Now, if you have a good number of these molecule, there is no reason why
the MHC-1 molecules would not express the antigenic peptide on the cell surface, which
would be recognized by the cytotoxic T cells of the same individual, which can recognize
cell MHC and thereby the peptide in the context of cell MHC and bring about killing. Now,
I have not shown how the killing? It is because of perforin and granzymes.
Now, what would happen if the MHC molecules start getting down regulated? CTLs can no
longer recognize and therefore, low MHC bearing tumor cells can go undetected. In fact, the
immune response itself may play a role in selecting out the low MHC bearing cells because
the strength of binding of the CTL to the peptide in the context of class 1 molecules
definitely starts to decrease with decrease in the number of MHC. Now, all the cells which
have tumor cells with high MHC expression would of course automatically get deleted.
So, thereby the immune response is actually is allowing or selecting those cancer cells,
which have low MHC molecules.
Now, this has been of course, shown in a number of cases that there is down regulation of
the MHC molecules. Another important mechanism by which cancer cells evade the immune system
is tipping the balance from TH1 to TH2 type of a response. I have not taught or lectured
on the T cells, but by now again, you would have, you know about the 2 types of helper
T cells, TH1 and TH2 and TH1 secretes cytokines, which help the cytotoxic T cells to proliferate
and TH2 help the B cells to proliferate. TH2 therefore, would secrete cytokines such as
interleukin4, which is required for B cell proliferation soon after activation.
Now, during cancer or Well, I can give you an analogy with respect to cancer pregnancy
and the foetus. Actually, the foetus is similar to a cancer. During pregnancy, the mother's
immune response has to be modulated to an extent that it does not attack the foetus
and you know the foetus is an allograft, 50 percent father 50 percent mother. In pregnancy
also, it has been seen that the TH2 type of response gets elevated and concomitantly TH1
goes down. In fact, there is a very good balance between the TH1. That is, the TH1 cytokines,
which help the cell mediated immunity versus TH2 which help the humoral immunity; now,
there is always a perfect balance. However, in certain cases like as said in
pregnancy, TH1 gets down regulated so that TH2 gets up regulated. The balance is always,
it is automatic. You have TH2 going down; you have TH1 going up. So, it is like a see
saw. So, during pregnancy, TH1 has to get down regulated because the T cell responses
or cell mediated responses to the foetal allograft needs to decrease.
Cancer and That is of course, under so called natural conditions. There are of course, large
number of factors which do this, you know tipping the balance in the favour of growth
of the foetus. In case of cancer, it has been seen similar situation takes place. There
are some factors which have not been really fully identified, but there have been factors
which are synthesized by the cancer cells such that there is an up regulation of TH2.
Now, as I have already mentioned a little while ago, presence of antibodies to the tumor
specific antigen does little to eradicate the tumor.
In fact, these antibodies can enhance. So, therefore, TH2 up regulation would only help
the tumor to grow better. So, in this case, especially in case of the tumor, an effective
immune response to combat cancer would be cell mediated rather than humoral immune response
and it has been shown adequately in animal experiments, animals which have tumors or
those that have progressively increasing tumors have an up regulation of the TH2 type of cells
and down regulation of TH1. So, again one more mechanism is that the cancer can tip
the balance from TH1 to TH2.
Now, this is not something, which probably the tumor cells do it actively like in the
previous cases, but the tumor cells may provide low co-stimulatory signals, but this could
be a mechanism by which tumor cells can escape the immune response. So, for proliferation
of T cells, you might remember, for proliferation of T cells, apart from activation signal,
which is through the peptide MHC recognition, through TCR or T cell receptor, there is a
competence signal required. So, for activation, it is only the recognition,
but for proliferation, a competence signal B7 on the antigen presuming cells and CD28
on the T cells, this kind of an interaction is required. If there are no APCs or antigen
presuming cells in the tumor vicinity, there would also be absence of co-stimulatory signals;
this would lead to clonal energy. So, this is again a way by which tumor cells which
have already started to decrease the MHC class 1 molecules, these go undetected.
Now, let us come to management of cancer. We have talked about antigens and the way
immune response is generated against cancer. We can look at now, management of cancer from
the view point of immuno therapy, but before we do that let us look at how cancer is managed.
That means what are the target molecules in the cancer cell, which can delete or kill
the cancer cell. So, the cancer therapy targets cellular processes that govern cell division
directly or indirectly. We do know that cancer cells are fast growing cells, which do not
confirm to the dictates of hematopoiesis or you know the inhibition of cell division,
when there are adequate number of cells that are already formed.
So, what are these molecules? Of course, DNA synthesis itself, drugs that inhibit DNA synthesis,
drugs that inhibit transcription, drugs that inhibit microtubule formation and drugs that
break DNA; all these are the basis of management of cancer. There are of course, others and
there are newer ones, but these are all the therapies that are still in practice and for
several long years, are those that in fact do not allow transcription and DNA synthesis,
thereby cell division itself is inhibited.
Radiotherapy and chemotherapy, both these are of course, still being used to control
cancer because these are effective in controlling cell proliferation. However, there is a reason
now, though chemotherapy and radiotherapy are extremely efficient, in inhibiting cell
proliferation, why cancer still persists? Because radiotherapy and chemotherapy, both
of them target fast proliferating cells. Therefore, the normal cells of our body, which also undergo
fast proliferation, they get affected and very seriously.
For example, cells of the hematopoietic cells, epithelial lining in the gut as well as sperms,
hematopoiesis itself is affected. No blood cells, no RBCs, no WBCs, all numbers very,
very low. Therefore, when especially chemotherapy, now radiotherapy of course, is little bit,
you know radiotherapy is given focused on the cancer itself. So, may be things are little
better, with but chemotherapy affects the entire body. They are agents, I mean medicine
that is taken in the body. So, treatment therefore, has to be withdrawn for recovery of general
health. Because you cannot try to kill the cancer
cells completely, if the individual is going to die because of anemia. Therefore, treatment
to eradicate cancer is done and withdrawn as soon as there are alarming signals of low
blood count and of course, toxicity because of the epithelial lining getting disturbed.
Now, when the treatment is withdrawn, there are residual cancer cells left, you know the
ones we have talked about, that down regulate the MHC molecules and I must add that cancer
cells can also, down regulate these tumor specification antigens with time. So, these
residual cancer cells start dividing and this leads to relapse.
Now, this has been shown. Well, I try to show this in this diagram. Typically you know This
is Y-axis, shows the tumor load and X-axis is the time. So, when the tumor load is very
high chemotherapy or radiotherapy is given constantly, in fact, there is a regiment of
this, the tumor immediately starts to decrease; the tumor load goes down. The patient now
is known to be in a state of remission. There are residual cancer cells which cannot even
be detected very easily. They are so low in number that even if you have biomarkers here,
you will not be able to detect. So, you know that this is the phase of remission.
There are residual cells here and we know that there are residual cells because the
moment that chemotherapy is withdrawn and has to be withdrawn here because of toxicity.
Toxicity, which is seen by way of lower number of blood cells, seen by way of the gut lining
getting totally destroyed, that leads to bleeding and well several other problems. So, now though
the person is in remission, soon after, number of tumors cells increase. Again there is treatment
given and the tumor load comes down; again there is a phase of remission.
But you know the phase cannot go further, remission cannot be better because of the
toxicity. Once again, now the tumor starts to proliferate. There would be of course,
slight difference between the tumor cells here verses tumor cells here. In fact, the
tumor cell starts to get a better and better with respect to managing, to evade the immune
system better and better. You can see also here, the first phase of remission is much
longer than the second phase and what you cannot see here that subsequently, if the
same cycle continues, the remission gets smaller and smaller and relapse occurs faster and
faster. Therefore, chemotherapy also has to be ultimately stopped.
Now, immunotherapy would be the method of choice here, to try to take away the residual
cells and this chemotherapy, in fact has started to gain importance. Now, of course, immunotherapy
becomes difficult with regard to, you know every tumor, every cancer cell is different,
but, you do know that chemotherapeutic agents as well as radiotherapy would now act on every
cancer cell. Immunotherapy has to be specific. So, let us look at where immunotherapy is
given? Can immunotherapy be given in the beginning, when the tumor load is high? It will not help
too much. However, it has been seen it is beneficial
to patients, when the treatment to get down the tumor's load is by chemotherapy or radiotherapy
and once the tumor load is very small, now, give the patient immunotherapy so that the
small number of cancer cells over here can be effectively recognized by the immunotherapeutic
agents, whatever one is given and destroyed and it has been seen that with such alternating
kind of therapy, chemotherapy and then immunotherapy, there are longer remission period and fewer
relapses; so, it is beneficial.
So, what are the cancer immunotherapy that are being utilized now and where research
is going on? Maybe to some extent, even in patients, it is still at the research stage.
One is manipulation of co-stimulatory signals. I have already told you that the lack of co-stimulatory
signals is in fact, what does not make an effective CTL response of cells - cytotoxic
T cell response and we do know that the cytotoxic T cells or the cell mediated immunity which
is better in eradicating cancer than antibodies. So, what people have tried to do is transfect
one's own tumor cells. Let us go back to, you know make it little simpler. Now, there
is a tumor in tumor load, There is a solid tumor in a patient. So, this tumor is regressed,
it is taken out, excised. Now, the tumor cells, you can make single cell suspension, transfect
these tumor cells with B7 so that now, there could be an effective, you know B7 binds to
CD28 of the cytotoxic T cells and this way, you can have an extremely good cell mediated
lysis. Secondly, enhancement of APC activity - antigen
presenting cell activity I told you is very important and this is what lacks in the vicinity
of several tumors. So, what people have done, taken the patient's dendritic cells, they
are cultured in cytokines such as GM-CSF - granulocyte, monocyte colony stimulating factor and also
incubated with tumor fragments. So, tumor fragments would have antigens which are now
taken up by the dendritic cells, then re-injected into tumor bearing mice and this activates
the TH and the cytotoxic T cells specific for the tumors.
So, you are activating the dendritic cells and by GM-CSF as well as tumor fragments,
re-injected effective APC activity. Cytokine therapy is quite well known. Cytokine therapy,
in fact, is in use in human. Gamma interferon alpha, beta; interleukin2, which would, you
know heighten the cell mediated immunity; 4, the B cells; 6, differentiating B cells;
GM-CSF, I have already talked about; TNF alpha, which brings about lysis.
The only negative thing about cytokine therapy is that each individual has to be tested for
several of these before one or a combination can be given because the cytokine network
itself is so complex that you know one starts to treat with one particular cytokine, it
would have repercussions on the entire network and what would be the outcome could be anybody's
guess, but any way anything to try to manage cancer. Fourth, in cancer immunotherapy and
which probably, you all must have already read quite a lot about are monoclonal antibodies.
There are eight different monoclonal antibodies that are being used which are commercially
available and are being used to treat cancers. Let us just quote. Rituximab to treat non
hodgkins lymphoma; Trastuzumab - breast cancer; Gemtuzumab - acute myeloid leukemia; Alemtuzumab
- chronic lymphocytic leukemia; Ibritumomab - non hodgkins lymphoma; so, also Tositumomab
- non hodgkins lymphoma; Cetuximab -colorectal, head and neck cancers; Bavacizumab - colorectal
cancer.
Now, what do all these Monoclonal antibodies do? Why Monoclonal antibodies? Because they
are specific to one particular epitope and one particular let us say, receptor or protein
on the cell. These antibodies can be, they can inhibit. For example, it is known that
Trastuzumab, this inhibits cell division because of inhibiting a particular signaling cascade.
The others inhibit ligand binding. There are those that can be used in combination and
let us look at that.
Now, apart from that in case of lymphomas, now, why are they three or well, three to
non hodgkins lymphoma and two to leukemias. Now, why monoclonal antibodies would be there
are, 5 out of 8 are two leukemias and lymphomas because again lymphomas are also well, they
are solid tumors, but not solid tumors as one would think in terms of carcinoma. It
is loosely associated mass of single cells, it is a lymphoma, but penetrance of the tumor
is very, very high. So, monoclonal antibodies also can penetrate.
So, these antibodies are specific to receptors, which are hyper expressed on these cancers
and these can evoke or fix complement rather easily because like I told you in case of
solid tumor, it is difficult to have tumor mediated lysis, but leukemias and lymphomas
much, much better and in fact, all these monoclonal antibodies have shown to be good with respect
to management of cancer.
So, anti-idiotypic mabs to lymphoma trigger complement mediated lysis; that means these
antibodies can bind to those specific receptors. there are people who have Now, this is another
type of therapy that people have tried to do and quite interesting really. Now, Levy
et al cloned and expressed immunoglobulin genes from lymphoma patient. Now, let us look
at how this would help. Immunoglobulin genes from lymphoma patients, now, that lymphoma
would actually be a monoclonal. I have told you that most cancers arise from
signal cell and therefore, are monoclonal. Therefore, the lymphoma of one patient would
have only one type of immunoglobulin hyper variable region. So, this research group,
they have cloned and expressed immunoglobulin genes from lymphoma patients. These immunoglobulins
would be specific to that lymphoma and so, conjugated to KLH - keyhole limpid haemocyanin
so that the immunogenicity is increased and now injected into patients.
They are large number of, if I remember correctly, there was something like 40 cases or more
studied and it was seen that anti idiotypic antibodies were generated to their own immunoglobulin
hyper variable regions in 50 percent of the cases and this thereby inhibited the tumor
growth and about 50 percent of this patients actually, the clinical outcomes are very beneficial,
quite interesting. This is of course, not a monoclonal antibody; it is monoclonal with
respect to the immunoglobulin genes that I express on the lymphoma patient cells because
the lymphoma is monoclonal. Mab Herceptin - this binds to HER2 receptors
on the tumor cells. This HER2 receptor are growth factor receptors. This stops the receptor
signaling and the cells to grow or divide. So, antibodies can also inhibit, if they are
specific to a particular growth factor. Fourth is making immuno toxins. That is, now, you
have monoclonal antibodies that can bind to tumors. Now, this might be quite helpful in
case of solid tumors; for example, where there is no antibodies bind to specifically tumor
specific antigens on the tumor cells, but there is no lysis because of lack of adequate
complement. So, instead, one can have monoclonal antibodies
that can recognize specifically, tumor cells or bind more to tumor cells. These antibodies
can be conjugated with toxins or chemical agents or radioactive molecules. Now, toxins
are elaborated a little bit more. Let us go to chemical agents. Chemical agents, why?
Because if you just inject chemotherapeutic agents, I have already told you that these
are going to affect also the normal cells and those that are rapidly growing, for example,
the cells of the bone marrow. Instead, suppose this chemical reagent is conjugated to the
antibody, then the antibody delivers specifically the therapeutic agent. Antibodies can also
be conjugated to radioactive molecules so that there is specific delivery.
Now, I would like to go to cell targeted immunotherapy using toxins. One reason for why I would like
to elaborate this is because we are doing research in this particular area. Cell targeted
immunotherapy using toxins - there are a large number of toxins which are used. Bacterial
toxins or plant toxins are well known. I will talk about plant toxins called ribosome inactivating
proteins or RIPs in short, which can be either of type 1 of type 2. The type 1 has only the
A chain, which is the toxin and type 2 has A chain, which is conjugated to the B chain.
Type 1, which has only the A chain does not in any way kill a cell, eukaryotic cell I
am talking about; so, therefore, mammalian cells as well.
The type 2 on the other hand, allows the entry of the A chain because the B chain is the
lectin. It binds specifically to sugars. In case of the one that we are using in my lab,
the B chain is a galactose specific lectin and eukaryotic cells have large number of
these galactose on the cell surface. So, any receptors which have terminal galactose, the
B chain binds to the entire molecule; therefore, gets internalized. A few molecules escape
the regular pathway, the lysosomal degradation and they go through a retrograde transport,
where the molecule goes first to the Golgi and then to the endoplasmic reticulum.
Here, there is cleavage between A and B chain because they are single disulphide bond which
gets cleaved in the ER. The B chain because it is a lectin, again gets retained on the
glycoproteins, which are present in the ER and the A chain is thrown out in the cytosol,
where it binds to a specific it binds to the loop structure in the 28S ribosomal RNA, depurinates
it, thereby inhibits the elongation factor from binding and protein synthesis is completely
stalled. The toxin, whether it is an A chain of the type 1 or A chain of the type 2, this
is deglycosidase; it is an enzyme. So, being an enzyme one single molecule has been reported
to inhibit 2000 ribosomes per minute. So, that is really extremely fast, you know highly
efficient enzyme; protein synthesis stalls and the cell dies.
Where do you find type 1 RIP and type 2? You know interestingly these are found in vegetables
[FL] momordica charantia, this has toxin called momordin. So, one can conjugate directly the
type 1 RIP and in case of type 2, you take away the B chain and you replace the B chain
with an antibody molecule. The antibody is specific and it would target only those cells,
which have the antigen to which the antibody is directed. Normal cells
Now, I have shown two situations. In case of a tumor specific antigen like for example,
those that are induced, that the tumor cell is induced by let us say, a virus. Then of
course, you would have those only on the cancer cells; you would not have those virally induced
antigens on normal cells. So, the normal cells are not touched at all. However, in tumor
associated antigen, there is a chance that you can have some normal cells also having
those antigens, but the number of antigens on normal cells would be minimal. So, you
have now, an antibody molecule which can recognize a cancer cell, only if it has x number of
antigens on the cell surface because of its affinity.
So, in such a situation of course, the antibody molecule would no way bind to the normal cell.
So, you have now, antigen bearing cancer cells, which are now being recognized by the antibody,
which is to the specific antigen. These antibodies conjugated to the A chain, the A chain, either
from type 1 RIP or the A chain from type 2 RIP, there is a single disulphide bond, which
is introduced, which is present here. So, the antibody molecule has to be slightly modified
so that you have an S group, a cysteine, which allows a linkage to the A chain.
This entire molecule then is internalized and what we are trying to see, what we have
shown already and others have shown that this immunotoxin is a hybrid between the toxin
and the antibody kills specifically cancer cells or those cells which have antigens on
the cell surface. Now, I will of course, talk about a little bit of details about this.
This is too simplified, but in any case, we have shown that it does not kill normal cells;
that means it does not get entry. Now, the question will be, will all antigens, all tumor
specific antigens upon binding to the antibody bring about internalization of the toxin because
of course, the A chain has to be taken in. So, what is important here for the generation
of immunotoxin, such an antibody has to be used, which has to be shown at first after
binding to its antigen, there is a internalization process which takes place, which is in fact,
energy dependent phenomenon. It is only when this whole molecule gets internalized, will
the toxin be able to do its job. So, this is quite a complex situation, but just see
how important beneficial it would be. If all the cancer cells, which have these receptors
or specific antigens, specifically singled out by way of these antibodies or immunotoxins
and killed; normal cells are left untouched.
So, the molecule, which is in clinical trials now, is a toxin called risen, which is similar
to the one that we are studying in my lab. It is type 2 RIP and people have in fact,
started to show that risen bound to antibodies, not only antibodies, also ligands to specific
receptors. These toxins are extremely efficient in taking away those residual cells, which
remain after the tumor burden is down. Alright, are there any other ways by which an immunotherapy
can be utilized? This is again on research stage, but extremely promising.
Fortifying the patient's immune system against his cancer - So, what is done here, T cells,
I told you that T cells or cytotoxic T cells would be a better bet to eradicate the cancer,
than antibodies. T cells are isolated from patient's blood and they are induced to proliferate
in defined medium outside the body. Now, why this? Because it has been shown that patients
or it has been shown also in case of mice, tumor bearing individuals seem to have an
immuno compromised state, even if the tumor is a solid tumor and not that of the blood
cells. Therefore, their T cells in the blood do not
proliferate, cannot be induced to proliferate as simple, if the T cells are isolated, induced
to proliferate in refined medium outside in an incubator under of course, regulated conditions
of 37 degree centigrade and in a CO2 incubator. After a good number has been generated, then
these can be stored in liquid nitrogen. Then, when the patient can be given chemotherapy
or radiotherapy, the tumor burden is now down. Then the same cells, which you believe has
a good population of these T cells, would have specific cytotoxic activity, can be injected
back into the patient when the tumor load is low. Remember, the dips. So, when the tumor
load is high versus when the tumor load has come down after chemotherapy and there are
residual cells, now, the residual cells, a smaller number of cells can be effectively
killed by the cytotoxic T cells from the patient, which have anyway been induced to proliferate
and they are larger in number. So, this shows well, how one can actually purify the mononuclear
cells.
Now, a step forward in this direction would be you know what I talked about so far are
lymphocytes which have been taken from the patient, which was sensitized in vivo. They
are just been allowed to grow out and then re-injected when the tumor load is low. 1
step better would be to take the lymphocytes taken from the patients the same way now by
this gradient centrifugation take the patient blood co-cultivate with the patient's own
cancer cells. so you take the mononuclear cells, lymphocytes in other words and now
you put them in culture along with the receptive tumor from the patient.
Of course, one needs to make single cells. These two are co-cultivated. So, in other
words, the patient's lymphocytes are being again taught to recognize or get activated
to the own cancer cells. There can be amplification or there is amplification of the T cells.
One can of course, add cytokines such as interleukin2 and then these can be stored once again. Why
store? Because if you inject the patient immediately after the tumor is resected, there will not
be much help, but after the patient has gained some slightly healthy condition, then the
viable cells, which have undergone training again can be injected and this has to be in
there are clinical trials that have gone to show that this has is of great benefit to
the patient.
So, combination therapy is absolutely, the method of choice. There is no one therapy
for cancer management; combination therapy where surgery, in case of a solid tumor or
chemotherapy, in case of lymphomas and leukemias; it can also be radiation therapy. So, first,
by surgery and chemotherapy or radiotherapy, the tumor load is brought down to a minimal
level. After this, when the tumor load is very low,
this is when a specific therapy can be introduced, either by way of, you know what I talked about
the immunotoxins, where there are already antibodies. One should have already had antibodies
specific to the tumor or receptors specific to the tumors, which are not there on normal
cells or if they are there, very low. Conjugate to toxin can be injected at this stage or
the T cells, either simply proliferated in vitro stored and then injected here or T cells,
which have been re-sensitized to the patient's own cells, the cancer cells I mean, re-injected
over here. Look at the progress of the disease. Now,
this kind of treatment seems to be working not very well, but at least has lent more
years to the cancer patients and also a healthier life. How does one know that now immunotherapy
has to be given again? Now, if given immunotherapy, the person is in remission. How one does know
when to give any therapy at all? Now, in case of very fast growing tumors may be the patient
might require chemotherapy again. But in case of slow growing tumors, one can
keep on monitoring by carrying out immunodiagnostics to find out when the cancers even in spite
of giving immunotherapy, has started to come back in circulation. Monitor, therefore, for
tumor markers. As soon as these tumor markers, for example, in case of liver cancer, you
can look for alpha beta proteins. As soon as the lowest detectable concentration of
alpha beta protein can be seen in circulation, immediately without waiting, another lot of
immunotherapy can be given. This way, one can effectively curtail cancer.
So, just like to summarize here. There is evidence that tumors have antigens. There
is evidence that these are recognized as non-self and an effective immune response is generated
to this; there is ample evidence. Unfortunately, in case of cancers, most cancers are unique,
especially if they are chemotherapeutically I am sorry, carcinogenically induced. If they
are virally induced, then maybe, there would be similarities with respect to tumor specific
antigens. However, the negative part about immunotherapy or the immune system, trying
to boost the immune system would be, to be able to have unique reagents that recognize
those unique antigens. Nevertheless, we do know that there can be
an immune response that can generated to the tumors. So, cancer patient can and is able
to mount an immune response to the tumor antigens. Of course, cancer cells have a mechanism to
evade the immune response. Therefore, the management of cancer would be to fortify the
immune response to kill tumor cells. And right now, the only way by which one can
effectively eradicate a cancer would be to have a combination therapy, where the major
part of lessoning the tumor burden is by non-specific mechanisms like chemotherapeutic agents or
radiotherapy and then come with the most specific response or specific therapy by way of the
immune response, which is allowing the own person's own immune system to effectively
recognize and not only recognize, kill the tumor cells. So, I will stop here. Thank you.