The Nonclassical Major Histocompatibility Complex Molecule Qa-2 Protects Tumor Cells from NK Cell- and Lymphokine-Activated Killer Cell-Mediated Cytolysis

Abstract

The cytotoxic activity of NK cells is regulated by class I MHC proteins. Although much has been learned about NK recognition of class I autologous targets, the mechanisms of NK self-tolerance are poorly understood. To examine the role of a nonpolymorphic, ubiquitously expressed class Ib Ag, Q9, we expressed it on class I-deficient and NK-sensitive B78H1 melanoma. Presence of this Qa-2 family member on tumor cells partially protected targets from lysis by bulk lymphokine-activated killer (LAK) cells. H-2Kb-expressing B78H1 targets also reduced LAK cell activity, while H-2Db offered no protection. Importantly, blocking with F(ab′)2 specific for Q9 or removal of this GPI-attached molecule by phospholipase C cleavage restored killing to the level of vector-transfected cells. Experiments with LAK cells derived from H2b SCID and B6 mice established that NK1.1+TCR− NK and NK1.1+TCR+ LAK cells were the prevalent cytolytic populations inhibitable by Q9. Treatment of mice with poly(I:C) also resulted in generation of Q9-regulated splenic cytotoxicity. LAK cells from different mouse strains responded to Q9, suggesting that the protective effect of this molecule is not detectably influenced by Ly49 polymorphisms or the presence/absence of Q9 in NK-harboring hosts. We propose that Q9 expressed on melanoma cells serves as a ligand for yet unidentified NK inhibitory receptor(s) expressed on NK1.1+ NK/T cells.

In vivo tumor cell rejection induced by NK cell inhibitory receptor blockade: Maintained tolerance to normal cells even in the presence of IL‐2

“…Second, other ligands of inhibitory receptors may confer protection of normal cells. Although Ly49C/I blockade did not induce rejection of spleen cells expressing H-2K b as the only inhibitory classical MHC class I ligand, whereas b2m À/À cells were rejected, it is possible that the former express non-classical, b2m-dependent molecules that could protect normal cells [38]. Such interactions could be investigated by expanding the blockade to other inhibitory receptors.…”

The Role of Structurally Conserved Class I MHC in Tumor Rejection: Contribution of the Q8 Locus

“…In addition, B78H1 cells are deficient in transcription of Ag processing components, such as TAP2 and LMP7, but express high levels of ␤ 2 -microglobulin chains required for assembly of mature class I MHC. Surface display of transfected Q9 in B78H1 cells was previously shown to depend on cotransfection of TAP2 (10).…”

“…and GM-CSF-transduced B78H1 expressing both Q9 and TAP2 (clone GMQ9TAP.13, designated as GMQ9TAP) or TAP2 alone (GMTAP) have been previously described (1,10), as have Q9-expressing 3LLA9F1 Lewis lung carcinoma (3LL-Q9) and RMA T cell lymphoma (RMA-Q9) transfectants (3). Q8-expressing B78H1 transfectants were derived by similar methods.…”

“…In vitro cytotoxicity assays 51 Cr release assays were performed as previously described (2,3,10). Briefly, effector cells at various E:T ratios in a volume of 100 l were added to the wells of a U-bottom 96-well plate.…”

“…This cell line is devoid of cell surface-expressed class I MHC due to the absence of H2-K b , H2-D b , Qa-2, and other MHC class Ib H chain transcripts (10). In addition, B78H1 cells are deficient in transcription of Ag processing components, such as TAP2 and LMP7, but express high levels of ␤ 2 -microglobulin chains required for assembly of mature class I MHC.…”