Two things
caught my attention: 1) The 5-year survival rate of all cancers has improved
from 49% to 68% in the past two decades with more than 19 million cancer
survivors expected by 2024 and the knowledge of how to manipulate the immune
system against the dreaded disease. We will discuss the latter further.
The hallmark
of scientific work seems to have been in the realm of Immunosurveillance and
cancer. The thrust of the argument for better control of the malignant tragedy
is based on the Adaptive Immunity against cancer.
The knowledge that there are
lymphocytic infiltrates found within the cancer milieu is well known for
decades. It was realized some 30 years ago in the colorectal cancer domain that
those patients who had lymphocytic infiltrates in their cancers did better with
stage, disease progression and overall survival. Now with the journey of these
lymphocytic infiltrates laid out, actionable sites have become visible for the
probing and assault against this deadly scourge.
The adaptive
Immunity to be effective has to have two components of T-lymphocytes: The T
Helper cells (Th1) and the Cytotoxic T-Cells (CTL). The CTL upon stimulation
secrete TNF-α and Interferon-ϒ, both of which have limited immunity. The Regulatory
cells are the T-regs that oppose the inflammatory signals via TGF-β and
Interleukin to suppress the CTL from further activity in order to limit immune
activity and tissue destruction. A balance therefore exists between the T-regs
and the Th-cells.
The
universal knowledge states that Immunity is based on the “Self” and the
“Non-Self.” The T-Cells are programmed to recognize and destroy any foreign
invaders. However as a means of checks and balances there is a proviso that
allows for some self-control to prevent an all-out attack against the “self” should
the invaders be contained. This mechanism is available via control-points also
called appropriately “Check-Points.”
The express function of modern-day immune
manipulation against cancer is to unbalance the inhibitory signals to the
checkpoints to allow for a full-throated attack against the wayward cancer
cells. Meanwhile, not to be outdone, the cancer cells have tricks up their
sleeves too and try to fool the Immune T-Cells with their vile secretory
expressions of TGF- β and Interleukins along with other co-stimulatory
cytokines like CD28, CD137 and OX40 to force the immune cells to lay down their
arms in quietude. This fight is worth a few more words.
As the
cancer cell invades tissues, it is also invaded by these immune cells. The
Immune cells have to recognize the cancer cell surface antigens (Major
HistoCompatibility Loci) and then” express their outrage” via their secretory
products to limit growth of these invaders. Three mechanistic offenses are
launched against the cancer cells:
Retaliation 1: The recruitment of the Th1 and CTLs
to gear for a limited response. This is followed by an assessment of the damage
against the enemy. If the immunity is weak this is the last signal and the
cancer enjoys free reign without any further intervention. In this scenario, for
example, therapeutic intervention with Herceptin and Chemotherapy are the
mainstay of therapy in breast cancer.
Retaliation 2: In this format, the initial
response is followed by ancillary assault via the co-stimulatory APCs and the
Checkpoint regulations. PD-, PD-L1,PD-L2, which are “programmed cell death”
inhibitors. Inhibition of these checkpoints asserts
limitations on the immune response and allows cancer cell growth. By inhibiting
the inhibitors, the immune attack continues. The PD-, PD-L1,PD-L2 molecules fit
the cell receptor sites on the CTLs and abrogate their ability to fight. Anti
PD-1, PD-L1 and PD-L2 antibodies therefore allow a new breed of T-Cells to
invade along with their co-stimulatory cytokines such as TNF- α and the
Interleukins to play havoc with the inflammatory phenomenon and thus destroy
the cancerous invaders. Checkpoint inhibitors and Monoclonal antibodies
directed against the tumor cell expressed proteins are the mainstay in this
approach. Disease such as Melanoma, Non-Small Cell Lung Cancer and Colorectal
cancer has been successfully treated with this form of immune modulation.
Retaliation 3: Inactivation of the TGF-Beta and
the Interleukins. The cancer cells co-opt the protein expression and inactivate
the inflammatory signaling secretions. Again the Checkpoint inhibitors and
Monoclonal Antibodies along with other conventional measures can be used with
impunity against the cancer.
Would you
agree that the knowledge of immune function activity in and around the cancer
has meaningful repercussions in our battle against this disease? We have yet to
determine the EMT (Epithelial Mesenchymal Transition) effect on Immune
modulation and that may hold more answers in the future. Is that a concept
worth exploring?
Checkpoints: Initial phase= CTLA-4 (Iplimumab-binds
to APC via CD28), Secondary phase when cytokines have been released then
PD-1(expressed by T-cells, NK cells),
PD-L1 (APCs and Epithelial cells), PD-L2 ligands inhibit CTL activity,
(Melanoma, NSCLC, CRC) LAG-3
Antibodies against Checkpoints and in
development: Iplimumab CTLA-4, Nivolumab PD-1, PD-L1(BMS-936559, PD-L2
(AMP-224, LAG3 (IMP321.
Speaking
about MoAbs, Ibrutinib against the Bruton Tyrosine Kinase against CLL is a big
story that we can discuss in the future.
References:
Scott N.
Mueller, PD-L1 has distinct functions in hematopoietic and nonhematopoietic
cells in regulating T cell responses during chronic infection in mice J Clin
Invest. 2010;120(7):2508–2515.
Shoba
Amarnath The PDL1-PD1 Axis Converts Human TH1 Cells into Regulatory T Cells.
Sci Transl Med 30 November 2011: Vol. 3, Issue 111, p. 111ra120
Sharpe AH,
Wherry EJ, Ahmed R, Freeman GJ. The function of programmed cell death 1 and its
ligands in regulating autoimmunity and infection. Nat Immunol.
2007;8(3):239–245.
Ahmed
Tarhini, Earnest Lo, David, Minor.
Releasing the Brake on the Immune System: Ipilimumab in Melanoma and
Other Tumors. Cancer Biother Radiopharm. Dec 2010; 25(6): 601–613. Article
online: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3011989/
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