Bovine NK and LAK susceptibility is independent of class I expression on B lymphoblastoid variants

Numerous tumors express low or no class I molecules, resulting in their avoidance of recognition and destruction by different effector cells of the immune system. Using a parent and two MHC class I mutant cell lines, we have tested the role of MHC class I molecules in natural killer (NK) cells, lymphokine activated killer (LAK) cells and cytotoxic T lymphocytes (CTLs). Both class I expressing parent cells and class I loss mutants were insensitive to NK cell lysis as assayed, regardless of the amount of class I molecules on the target cell surface. However, LAK cells demonstrated higher cytolysis on these target cells than NK cells, suggesting different mechanisms of target cell recognition or different levels of lytic activity by these two effector cell populations. Up-regulation of class I expression on the target surface by gamma interferon (gamma-IFN) had little influence on NK and LAK susceptibility, indicating there was no correlation between class I expression and bovine NK or LAK cytolysis. However, allogeneic CTLs mediated a lytic pattern distinct from NK and LAK cells, in which target sensitivity to allogeneic CTLs correlated with the amount of class I molecules expressed on the cell surface. Additionally, effector-target cell conjugation studies demonstrated that target class I expression was not involved in NK and LAK cells binding to targets. These results demonstrate that NK and LAK cytolysis of these two class I mutant cell lines is independent of the amount of class I molecules expressed on the target cell surface.