An Immature Rat Lymphocyte Marker CD157: Striking Differences in the Expression between Mice and Rats

Abstract

We have established a novel monoclonal antibody that recognises mouse and rat CD157, and uncovered striking differences in both the level and stage of expression of this antigen in the primary lymphoid organs between these two species. Unlike mouse, the majority of rat thymocytes express CD157. SHR and WKY rats were the exception, having unusually low levels (similar to those of the mouse) of these cells. However, in both species, a subset of CD3 -CD4 -CD8 - thymocytes exhibited high levels of CD157. Surprisingly, these CD157 high cells temporarily upregulated MHC class I molecules in both species. Furthermore, a third of CD157 high rat thymocytes were CD45RC +, a marker found on immature thymocytes with regenerative capacity. Examination of the bone marrow lymphoid population shows that the expression of rat CD157 is largely observed at the CD45R +IgM - pre-B-II cell stage, and unlike mouse, extension of expression into the IgM + immature B cell stage was marginal. Similar to CD157 high immature thymocytes, these immature B cells also expressed high levels of MH C class I. With the exception of the LEC, SHR and WKY rat strains, which have three- to four-fold less CD15 + bone marrow myeloid cells, percentages of these cells are similar between these two species. Thus, marked differences in the level and stage(s) of CD 157 expression on lymphoid cells in mouse and rat indicate that CD 157 may not, as previously thought, have a direct role in T or B cell differentiation.