A discrete 10-kDa polypeptide (10K) is expressed from early stages in the embryonic chicken lens. Since this has potential as a marker for lens cell development, chicken 10K and its homologues from mouse and human lenses were identified by protein sequencing and cloning. Surprisingly, lens 10K proteins appear to be identical to a lymphokine, macrophage migration inhibitory factor (MIF), originally identified in activated human T cells. Using microdissection and PCR techniques, we find that expression of 10K/MIF is strongly correlated with cell differentiation in the developing chicken lens. Northern blot analysis shows that 10K/MIF is widely expressed in mouse tissues. These results suggest that proteins with MIF activity may have roles beyond the immune system, perhaps as intercellular messengers or part of the machinery of differentiation itself. Indeed, partial sequence of other small lens proteins identifies another MIF-related protein (MRP8) in calf lens. The relatively abundant expression of MIF in lens may have clinical significance, with the possibility of involvement in ocular inflammations that may follow damage to the lens.

The status of major histocompatibility complex (MHC) class I and II gene expression in the normal mouse lens was examined. No mRNA for either class I or II genes was detectable in mouse lens, while the expression of MHC genes in other tissues generally matched immunohistochemical data from human tissues. However it was observed that MHC class I mRNA is present in the mouse lens-derived cell line alpha TN4-1. From a new-born mouse lens cDNA library a clone was obtained for the murine homologue of the DNA-binding protein dbpB/YB-1, a protein originally identified in human lymphocytes and proposed to be a negative regulator of MHC class II gene expression. Northern blots detect dbpB/YB-1 mRNA in all mouse tissues and cells examined, including both mouse lens and alpha TN4-1 cells, suggesting that dbpB/YB-1 has a general and widespread role.