Correction of Defects Responsible for Impaired Qa-2 Class Ib MHC Expression on Melanoma Cells Protects Mice from Tumor Growth

Abstract

One of the principal mechanisms of tumor immune evasion is alteration of class I MHC expression. We have identified defects contributing to down-regulation of class I MHC expression in the widely studied murine B16 melanoma and its variants B16F1, B16F10, BL6-2, BL6-8 and B78H1. Transcription of the nonclassical class I MHC genes Q8 and Q9 (Qa-2 Ags) has been switched off in the entire panel of melanoma lines, suggesting that this event occurred early during tumor progression. B78H1, unlike B16F1 and B16F10 sublines, is also selectively devoid of TAP2 and low molecular weight protein 7 as well as classical class I MHC Kb and Db transcripts. Cotransfection of B78H1 with TAP2 and class I H chain genes is sufficient to reconstitute surface expression of exogenously delivered class I MHC without concomitant re-expression of endogenous β2-microglobulin-associated class I. The serological absence of endogenous class Ia and Ib at the surface of TAP2-negative as well as TAP2-transfected B78H1 makes this system a suitable model for studying the properties of isolated class I proteins in tumors. We used this system to demonstrate that B78H1 cells genetically manipulated to re-express Q9 Ag have reduced tumor potential in syngeneic B6 mice compared with TAP2-transfected parental melanoma. Both NK cells and CTLs appear to collaborate in restraining growth of Q9-positive tumors. The results implicate Qa-2 in antitumor responses and illustrate the utility of the B78H1 system for identifying in vivo interactions between class I MHC molecules of interest and immune cells of innate and/or adaptive immunity.

Synergistic Activation of Innate Immunity by Double-Stranded RNA and CpG DNA Promotes Enhanced Antitumor Activity

“…The B16-F 10 model of experimental pulmonary metastasis is poorly immunogenic as a result of down-regulated MHC expression and antigen processing similar to some human tumors (42,43). However, immunization of mice with tumor-associated antigens (TAAs) using pSIN, as well as other vaccination strategies, is capable of breaking immunological tolerance to B16-F 10 , resulting in acquired responses to melanoma TAAs (5,44).…”

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

“…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.…”

“…Tumor challenge experiments were performed as previously described (1)(2)(3). Briefly, 1 ϫ 10 5 live tumor cells were injected s.c. in a volume of 200 l of HBSS into the right rear flank.…”

“…Peptide-binding assays were performed as previously described (1). Briefly, cells were cultured in the presence or absence of peptide present at a concentration of 50 g/ml.…”

“…R ecent studies in our laboratory demonstrated that Q9, a member of the Qa-2 family of Q region-encoded nonpolymorphic class Ib MHC Ags, orchestrates strong antitumor immune responses against several syngeneic tumors, including the B16 melanoma variant B78H1, 3LL Lewis lung carcinoma, and RMA T cell lymphoma (1)(2)(3). Transcription of the Q9 gene is generally silenced or significantly down-regulated in tumors arising in vivo in Qa-2-positive H2 b or H2 d haplotype mice (4).…”