Immunogenetics 20: 603-622, 1984 Imllll O- genetics © Springer-Verlag 1984 Syrian Hamsters Express Two Monomorphic Class I Major Histocompatibility Complex Molecules Alix Gerboth Darden II and J. Wayne Streilein* The Graduate Program in Immunology and the Departments of Cell Biology and Internal Medicine, University of Texas Health ScienceCenter at Dallas, 5323 Harry Hines Boulevard, Dallas, Texas 75235 Abstract. The description of the Syrian hamster major histocompatibility complex (MHC), Hm-l, has progressed to the point that multiple class II alloantigens have been defined using structural and functional studies. However, no comparable success has been achieved using allotypic differences to detect class I molecules. We now report that xenoantisera raised in other species against hamster tissues have made it possible to describe class I MHC homologues in the hamster. Evidence which confirms that these molecules exist includes (1) on immunoprecipitation of radiolabeled lymphoid cell lysates, heterodimers of approximate molecular weight 47 000 and 12 000 are identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the latter representing hamster fi2-microglobulin; (2) by direct immunoprecipitation these molecules are ubiquitously expressed on hamster tissues; (3) partial N- terminal amino acid sequence analysis reveals striking homology with class I molecules described in other species. In addition, the amino acid sequence data reveal that two class I molecules are expressed on the surfaces of hamster cells. On two-dimensional PAGE analysis, these molecules are invariant among the several strains of genetically disparate hamsters available for study. We conclude that (1) hamsters have the capacity to make class I MHC molecules, (2) at least two genetic loci are dedicated to this purpose, and (3) no allelic forms can be detected, suggesting that there is no class I polymorphism. Address correspondence to: Dr. Alix G. Darden at her current address (see below). I[ Current address: Department of Internal Medicine, Royal Victoria Hospital, 687 Pine Avenue West, Montreal, Quebec, Canada H3A1A1 *-Current address: University of Miami School of Medicine, Department of Microbiology and Immunology, Miami, FL 33101 Abbreviations used in this paper: BM, bone marrow; BSA, bovine serum albumin; BSS, balanced salt solution; Con A, concanavalin A; IEF, isoelectric focusing; LPS, lipopolysaccharide; MHC, major histocompatibility complex; NP-40, nonidet P-40; PBS, phosphate buffered saline; S. aureus, Staphylococcus aureus; SDS-PAGE, sodium dodecyl sntfate polyacrylamidegel electrophoresis; TBS, Tris buffered saline; 2D-PAGE, two-dimensionalpolyacrylamidegel electrophoresis.