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Luteal cells express class II major histocompatibility complex (MHC) molecules and can stimulate T lymphocyte proliferation in vitro. However, it is unknown whether luteal cells express the intracellular components necessary to process the peptides presented by class II MHC molecules. The objective of the present study was to examine the expression and regulation of three major class II-associated antigen processing components – class II MHC-associated invariant chain (Ii), DMa and DMb – in luteal tissue. Corpora lutea were collected early in the estrous cycle, during midcycle and late in the estrous cycle, and at various times following administration of a luteolytic dose of prostaglandin F2a (PGF2a) to the cow. Northern analysis revealed the presence of mRNA encoding each of the class II MHC-associated antigen processing proteins in luteal tissue. Ii mRNA concentrations did not change during the estrous cycle, whereas DMa and DMb mRNA concentrations were highest in midcycle luteal tissue compared with either early or late luteal tissue. Tumor necrosis factor-a (TNF-a) reduced DMa mRNA concentrations in cultured luteal cells in the presence of LH or PGF2a. DMa and DMb mRNA were also present in highly enriched cultures of luteal endothelial (CLENDO) cells, and DMa mRNA concentrations were greater in CLENDO cultures compared with mixed luteal cell cultures. Expression of invariant chain, DMa and DMb genes indicates that cells within the corpus luteum express the minimal requirements to act as functional antigen-presenting cells, and the observation that CLENDO cells are a source of DMa and DMb mRNA indicates that non-immune cells within the corpus luteum may function as antigen-presenting cells.