Downregulation of MHC Class 1 molecules reduce Cytotoxic T Lymphocyte ability to identify tumor cells. the mouse models aggressive NKG2D-sensitive tumors arose in NKG2D deficient mice but also in NKG2D WT mice where there was evidence of active NKG2D-mediated immunosurveillance. This suggests that a fraction of lymphoma cells escaped by employing an alternative mechanism of NKG2D-mediated immunosurveillance evasion or simply a rapid growth rate.[17] Possible mechanisms for evasion include shedding of NKG2D ligands or inactivation of effector cells.[18, 19] Adaptations such as the shedding of NKG2D ligand reduces the immunogenicity of the tumor cell and may also have distant effects potentially downregulating NKG2D on effector cells further abrogating the NK/CTL antitumor response, although the significance of this mechanism is not fully determined.[1] Alternatively, aggressive tumors may overwhelm NKG2D-mediated killing whilst remain sensitive. Further to the Eu-gene mutations leading to loss of expression of the HLA class 1 complex.[23] Classical Hodgkin Lymphoma provides an example of both NKG2D-L shedding and suppression of the effector cell. Enzymes able to shed NKG2D-L from cell membrane are expressed on Reed-Sternberg (RS) cells and mesenchymal stromal cells (MSC) and shed ligand has been detected in supernatant from both cell types. RS cells lacking NKG2D-L are resistant to CTL killing and sensitivity is partially restored with upregulation of NKG2D-L expression.[18] Additionally, investigators noted that after co-culture with MSCs, cytolytic activity ADU-S100 (MIW815) against NKG2D-L+ cells was reduced apparently due to local TGF- production leading to NKG2D downregulation upon T lymphocytes.[18, 24] Other examples include Adult T-cell Leukaemia/Lymphoma (ATLL) ADU-S100 (MIW815) where interactions with epithelial cells lead to downregulation of NKG2D-L and evidence of downregulation on multiple T and B cell lymphoma lines. [25, 26] Open in a separate window Figure 2 Lymphoma evasion mechanisms undermine the immunosurveillance response1. NKG2D ligand shedding overwhelms NK cells leading to ADU-S100 (MIW815) downregulated NKG2D and a hypofunctional phenotype and reduced CD58 expression may impair ability for missing self recognition. 2. Downregulation of MHC Class 1 molecules reduce Cytotoxic T Lymphocyte ability to identify tumor cells. 3. If DAMPs are not present, dendritic cells present antigen without costimulatory molecules promoting immune tolerance and regulatory T phenotypes. 4. Indoleamine 2,3-dioxygenase (IDO) expression on tumor and tolerogenic dendritic cells impairs CTL activity ADU-S100 (MIW815) 5. TGF secretion and myeloid-derived suppressor cell (MDSC) activity further skews towards tolerogenic phenotypes. Natural Killer cells play an indispensable role in antitumor immunity through NGK2D-mediated activity and also their ability to recognise and kill cells which are missing self-antigen. However, early tumor development in RAG-deficient mice with no functional B, T or NKT cells and observations that lymphomagenesis risk in primary immunodeficiency is more closely related to T-cell number and dysfunction as opposed to immunodeficiency type demonstrates that innate antitumor activity alone is not sufficient for effective immunosurveillance.[8, 27, 28] Cytotoxic T Lymphocytes: the principal effector cell in anti-tumor immunosurveillance Cytotoxic T Lymphocytes play a central role as effector cells in tumor immunosurveillance (reviewed [29]). CTLs primarily identify cells with malignant potential through recognition through the T cell receptor of antigen presented through HLA class 1 complexes and target cells via 2 mechanisms, TNF receptor superfamily members 6 and 10 (TRAIL and Fas/CD95) or the perforin ADU-S100 (MIW815) and granzyme pathway.[29] Effective perforin-mediated cytotoxicity is important to CTL function. Perforin deficiency in mice leads to impaired control of transplanted lymphoma lines and increased Rabbit polyclonal to FASTK rates and early tumorigenesis.[30] Perforin deficient mice have been noted to have a 1000-fold increased risk of lymphoid malignancy.[31] Severe perforin gene defects usually present early with aggressive haemophagocytic lymphohistiocytosis but patients with a less severe phenotype present later or have susceptibility to lymphoma.[32] EBV-positive Hodgkin lymphoma has been reported in an individual with biallelic STXBP2 mutations, a gene required for perforin-containing lytic granule exocytosis and in a separate study 8 of 29 patients diagnosed with lymphomas with features of HLH harboured mono- or biallelic mutations of the perforin gene.[33, 34] However, resistance to perforin-mediated cytotoxicity is not usually an important mechanism in lymphomagenesis and most lymphomas remain sensitive to its effects.[35] Adaptations to suppress CTL function play a more important role in immune evasion. In contrast, evasion of CD95-mediated apoptosis frequently plays a role in lymphomagenesis and is achieved by a number of mechanisms. These include downregulation of surface receptors expression, secretion of soluble forms of CD95 and DcR3 (a soluble decoy receptor which binds CD95-L inhibiting CD95/CD95-L ligation).