34.1 Evasion of immune response by tumors

Immune evasion by tumors can be divided into intrinsic or extrinsic mechanisms based on how these mechanisms are mediated.

34.1.1 Intrinsic mechanism of immune evasion by tumor cells

•  Tumor cells are highly prone to mutation due to increased mitotic rate and hence the antigen expression may change at times and within a particular period tumor cells may lack or be deficit of the antigens that generate immune response.

•  Immune system may not have access to the tumor cells because of antigen masking effect of glycocalyx molecules. Glycocalyx molecules help in hiding the tumor cells from the immune system by getting expressed in higher amounts in tumor cells.

•  Tumor cells may prevent the immune response by associating with molecules that destroy the immune system.

•  Some of the released products from the tumor cells may prevent the immune response. e.g TGF-β inhibits lymphocytes and macrophages.

•  Tumor cells do not express class II MHC molecules so they may not elicit effective T cell immune response.

34.1.2 Extrinsic cellular suppression of anti-tumor immunity

•  M2 macrophages are associated with progression of tumor growth and thus these macrophages may suppress the T- cell response to the tumor cells.

•  T- Cell immunity can be lowered by the presence of regulatory T cells.

•  Myeloid – derived suppressor cells (MDSCs) are detrimental to T cell response and may recruit regulatory cells to suppress the immune response.