Membrane-bound versus soluble major histocompatibility complex Class I-related chain A and major histocompatibility complex Class I-related chain B differential expression: Mechanisms of tumor eradication versus evasion and current drug development strategies.

Major histocompatibility complex Class I-related chain A/chain B (MICA/MICB) is stress-inducible, highly polymorphic ligands whose expression at the transcript level has been detected in all tissues except the central nervous system. However, their restricted protein expression is due to their regulation at the posttranslational level. Its levels are elevated in virally infected and neoplastically transformed cells. Membrane expression of this NKG2DL marks the aberrant cells for elimination by those immune effector cells that express the cognate NKG2D receptor. Among the evasion strategies developed by tumors, the metalloprotease-dependent shedding of MICA/MICB from tumors (either the free or the exosome form) can contribute to the inhibition of cytolysis by the immune effector cells (all NK cells, most NKT cells; γδ CD8+ T cells and αβ CD8+ T cells, as well as some αβ CD4+ T cells). There are micro-RNA clusters that regulate surface expression and shedding. Polymorphic variants can be used as susceptibility/associative markers and can also possibly be used to correlate with tumor survival as well as staging/grading of tumors. Variations in the expression level require quantification of this marker for diagnostic/prognostic and therapeutic purposes. Mechanism-based studies would provide a better tumor-specific understanding of their relative roles in the processes of tumor cell elimination versus growth and progression. Last but not least, conventional, interlaboratory validated assays (for, e.g., antibody-based methods) should be replaced by robust, reproducible, feasible biophysics-based methods using tumor biopsies. Further, correlative DNA polymorphism-based studies can be done using biological fluids (for, e.g., human saliva) that can be sampled by minimally invasive means.