This study provides a basis for understanding the wide variations reported in the literature in IFN-gamma inducibility of class II MHC antigens on murine beta cells. Inducibility is not an intrinsic property of all mouse beta cells, but instead depends upon strain- (and tissue-) specific response modifying factors. This was demonstrated by comparison of constitutive and IFN-gamma-induced class I and class II MHC gene products on cultured islet cell monolayers. Islet cultures were established from autoimmune diabetes-prone NOD/Lt mice, diabetes-resistant NON/Lt and CBA/J mice, as well as F1 hybrids between these latter two strains and NOD/Lt. Cultures of peritoneal macrophages (M phi) from each strain were established as controls. After 3 wk of culture (with incubation in the presence or absence of IFN-gamma during the last 6 d), constitutive expression as well as IFN-gamma induction of class I MHC antigen expression was demonstrated on NOD/Lt and NON/Lt islet cells by antibody plus complement-mediated cytotoxicity. Although CBA/J islets and M phi did not maintain constitutive class I or class II antigen expression in culture in the absence of IFN-gamma, class I H-2Kk antigen was IFN-gamma inducible. Whereas IFN-gamma-induced class II I-Ak antigen on CBA/J M phi, it failed to induce this antigen on CBA/J islets. In contrast, I-A antigens were IFN-gamma inducible on NOD/Lt and NON/Lt islets and M phi. In (CBA x NOD)F1 hybrids, loss of IFN-gamma inducibility of the I-ANOD product established that suppression was mediated by a trans-acting factor from the CBA/J genome. In the course of these studies, IFN-gamma inducibility of a crossreactive occult class I-like antigen on both NOD/Lt islet cell and M phi cultures was unexpectedly detected when mAb 28-13-3 (public specificity 39, reactive with H-2Kb,f) was used as a negative control. Although not detectable by cytofluorographic analysis of freshly isolated NOD/Lt splenic leukocytes, occult antigen could be induced on NOD/Lt peritoneal macrophages (M phi) cultured for 3 d in IFN-gamma. Time course of induction showed the occult antigen to be distinct from NOD/Lt class I and II gene products. In both islet cell and M phi cultures established from (CBA x NOD)F1 hybrids, trans-suppressive factor(s) from the CBA/J genome not only suppressed IFN-gamma-induced expression of I-ANOD, but additionally suppressed occult antigen induction. Backcross of F1 to both parental strains indicated that the occult locus was on Chr 17, tightly linked to MHC.(ABSTRACT TRUNCATED AT 400 WORDS)

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