Cancer Therapy and Pathophysiology

In the past, immunosuppression by T-cells has been reported as a major obstacle for immunotherapy. Reduction of T-cell trafficking, differential functioning and signalling is the basis of novel therapeutic strategies for cancer.

T-regulatory cells in human tumours: Investigations by Sakaguchi and Curiel suggest that failure of anti-tumour immunity is related to suppression of TAA reactive lymphocytes by mediation of T-regulatory cells. Investigations also revealed the presence of T-regulatory cells in peripheral blood stream in all types of cancer which could be the reason for reduced TAA specific T- cell immunity in cancers rather than the suppressive activity of T-regulatory cells in tumour microenvironment as in case of auto-immune diseases.

Thymus derived natural TReg cells and locally induced TR1 cells are found in tumour microenvironment. There are four potential sources for their presence which are shown below diagrammatically (Weiping, 2006):

Source: Figure adapted from (Weiping, 2006).

• Trafficking: The TReg cells which are CD4+CD25+FOXP3+  emerge from the thymus, lymph, bone marrow and peripheral blood stream and travel towards the tumour. These cells express CCR4 (Chemokine receptor 4) and its ligand, CCL22 in the microenvironment of tumor. Formation of these cells stimulates the infiltration of TReg cell tumour.

• Differentiation: The presence of cells in the tumour microenvironment suppresses antigen presenting cell (APC) differentiation and function. These APCs which failed to differentiate, stimulates the differentiation of regulatory T-cells. IL-10 is also present in tumour microenvironment which is responsible for the differentiation of CD4+IL-10+TGFβ+ regulatory T cells and induction of TR1 cells.

• Expansion: Expansion is caused by dendritic cells in tumour microenvironment and lymph nodes.

• Conversion: High levels of TGFβ present in tumour microenvironment converts the normal T cells into T regulatory cells.

Suppressive mechanisms of regulatory T-cells: According to Weiping, there are four mechanisms addressing suppression of regulatory T-cells. They are:

a) Induction of B7-H4 expression and T-cell arrest: APCs induce B7-H4 expression through Treg cells and these B7-H4+ APCs stimulate T-cell cycle arrest.

b) APC and T-cell apoptosis: The TReg cells which are thereby activated cause apoptosis of T-cells and APCs through perforin or granzyme B dependent pathway.

c) Supression of T-cell activation by Tryptophan: T-lymphocyte associated antigen 4(CTLA4)+ induce IDO (indoleamine 2,3-dioxygenase) expression through APCs and this expression in turn suppresses activation of T-cells.

 

d) Release of Interleukin-10: IL-10 is released by T-regulatory cells and this inturn causes release of TGFβ. This directly causes inhibition of T-cell activation and causes APC suppression.

These multiple suppressive mechanisms act in vivo and cause temperament of TAA specific immunity. All the suppressive mechanisms are depicted diagrammatically above.

Some common questions posed in this context could be regarding the TAA specificity of Treg cells and acquirement of TAA specificity in tumours. Treg cells are specific or specific antibodies and Treg cells recognise mutated tumour antigens.

TARGETING REGULATORY T-CELLS IN CANCER:

This novel approach is based on the concept of reversing immunosuppression in cancer. There are numerous approaches for selective targeting and these approaches include the following:

1. CTLA4 Blockade: When CTLA4 was blocked invivo, immunity of tumour was improved (Van elsas et al., 2001; Sutmuller et al., 2001). Several clinical trial studies were conducted to study the effects of injecting CTLA4 specific antibody in patients. Treatment with specific antibodies leads to cancer regression in some patients. It also recorded auto-immune responses in certain population of patients. However this therapy did not include depletion of T regulatory cells in peripheral blood stream nor depleted FOXp3 mRNA. These investigations revealed that blocking CTLA4 signals could be a potential target of treating various cancers.

2. CD25-T cells Blockade: Another potential treatment approach aimed at depleting CD25-T cells by treatment with CD25-specific antibodies. Injecting antibodies of this kind reduced the number of regulatory T cells and caused improved tumour immunity in cancer patients. Ontak, a denileukin dititox is a fusion protein containing IL-2 combined with diphtheria toxin. This approved drug is used in some patients suffering from T-cell leukaemia. In these patients, the toxin caused hindrance to protein synthesis and lead to apoptosis. Single doses of this toxin reported the increase of T-cell activation in a group of patients. The main effect of CD25 –T cells is apoptosis and reduction of T-regulatory cells in the tumour microenvironment. This approach revealed the same results when investigated in different types of cancers concluding the same result. It is also reported that use of the toxin had variable effects on FOXP3 and mRNA expression in CD4 T cells in melanoma patients.  

There are many potential problems associated with this therapy such as the non appropriate therapy in certain stages of cancer where it tends to be ineffective. Clinical efficacy of the route of administration of these depleting agents must be studied before administration. Systemic administration leads to reduced bioavailability and reduced depletion. The most important problem is depletion of other important immune cells responsible for maintenance of immune functions in the body. Although depletion of T regulatory cells tends to be a good approach, studies are still under research for optimal treatment. Research is conducted to determine optimum treatment which includes appropriate dose, appropriate clinical doses for exhibiting required therapeutic effect and use of optimum dose to exhibit potential benefits.

3. Potential strategies to control T-reg cell function: These potential strategies involve targeting molecules in T-reg cells trafficking. In ovarian cancer, blockage of CCL22 causes reduced T-reg- cell tumour trafficking in tumour microenvironment (Curiel et al., 2004). Therefore one of the possible strategy is T-reg cell tumour trafficking. The second alternate possible strategy is alteration of t-reg cell differentiation and functioning. This is possible by blocking FOXP3, which is essential for T-reg cell functioning and differentiation. Other essential molecules involved in efficient functioning and differentiation of T-reg cells are IL-2, GITR and B7-H4. Therapeutic agents who block all these molecules serve as an essential strategy to reduce the differentiation of T-reg cells. All these strategies are used along with current potential treatment approaches for maximum benefit of the patients.

Immunotherapy and current methods of treating cancer is not considered up to the mark and one of the main reason which accounts for this is that immunosuppressive mechanisms discussed above are seen maximum in terminally ill cancer patients and not in early stages of cancer (Dun et al., 2004; Zou et al., 2005). Another possible cause which is accounted is that certain immunotherapy procedures promote regulatory T cells and tumour trafficking instead of depletion and selective improvement of antitumour effects. Care should be taken in this regard when treatment options are considered.

4. IL-2 Treatment, A failure of potential therapeutic target: In certain patients suffering from leukaemia, metastatic melanoma and renal cell cancer, IL-2 treatment has been given and long term treatment with IL-2 reported decrease in proliferation of T-cells, increase of CD24 and CD25- T cell numbers upon activation inside the body (Sereti, 2004). W.Z et al reported increased number of Treg cells when administered in patients suffering from renal cancer, Ewings sarcoma, alveolar rhabdomyosarcoma. IL-2 also led to stimulation of CXCR4, a chemokine receptor 4 and CCR4 expression on T reg cells. This expression caused promotion and migration of Treg cells to tumour micro environmental CXCL12 and CCL22. These investigations proved IL-2 contribution in T-reg cell differentiation, maintenance, expansion and suppressor activity. These findings indicate that the potential target has to be revised in cancer patients and cytokine therapy with IL-7, IL-15 and IL-21 should replace interleukin Il-2 therapy. The failure of this potential target has proven path for the research of other novel immunotherapeutic agents.

THERAPEUTIC TARGETING OF SUPRESSIVE MECHANISMS, COMBINATORIAL THERAPY:

After cancer is diagnosed by clinical or pathological diagnosis, patients are initially subjected to traditional tumour therapy which includes surgical debulking, chemotherapy, antitumour angiogenic therapy and radiation therapy depending upon the stage of cancer. Individual treatment or a combination o either o these treatments is suggested. In this kind o treatment, tumour is itself targeted and considered as a standard therapy. This therapy has some disadvantages of side effects and is considered to be slow.

On the other hand, In conventional therapy, supplements of the immune system like tumour associated antigens (TAA), immune elements, antigen presenting cells (APCs), effector T cells, cytokines and chemokines are supplied with the aim to boost TAA specific immunity. These trials of therapy are improving patient conditions but constant improvement is needed.

In novel therapy, therapeutic strategies are used to target either regulatory T cells or suppressive target molecules. The therapeutic strategies which aim at regulatory T cells include

• Depleting (Denilekin digitoxin, a CD25- specific antibody and cyclophosphamide).  
• Blocking trafficking by CCL22- specific antibodies.
• Blocking the differentiation and signalling by blocking FOXP3 signals.

Targeting the suppressive molecules include:

• Blocking suppressive molecules like B7-H1, B7-H4, IDO, arginase on APCs.
• Blocking CTLA4, PD1 molecules on T cells.
• Blocking soluble suppressive molecules like TGF-β, COX2, IL-10.

Hence, to attain effective, reliable and quality in clinical management, a combination o traditional tumour therapy, conventional immunotherapy and novel tumour immunotherapy is necessary.

CONCLUSION:

Research conducted over the years proves that tumours develop mechanisms to withstand immunity and overcome conventional immunotherapeutic strategies. Regulatory T cells play an important role in suppression of TAA specific immunity and manipulation of these T regulatory cells serves as an important component in new therapeutic strategies. Modification of Treg cells includes blocking trafficking, reduction of suppression mechanisms and reduction of differentiation. In certain cases o tumour stages, modification of T regulatory cells is not the only suppressive mechanism and for this particular stage, studying molecular basis and nature of T regulatory cells becomes essential.

Therapeutic agents that target the T regulatory cells need extensive research to develop novel therapeutic strategies. Even with the success of novel therapeutic approaches, a combinational therapy involving old and conventional therapeutic strategies is encouraged.

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