Extreme skewing of annexin II and S100A6 expression identified by proteomic analysis of peritoneal B-1 cells

B-1 cells differ phenotypically, biochemically and functionally from conventional B-2 cells. The origin of these differences remains uncertain. We hypothesized that unbiased analysis of differences in protein expression between B-1 and B-2 cells might provide information not otherwise available, and thus undertook 1-dimensional (1D) gel analysis combined with mass spectrometry. We identified annexin II and S100A6 in peritoneal B-1 cells (B-1P) but not in splenic B-2 cells (B-2S); these results were confirmed by western blot analysis and reflected in mRNA expression. Further analysis of mRNA indicated that elevated expression levels of annexin II and S100A6 were unique to B-1P among several naive lymphoid populations. However, expression of annexin II and S100A6 protein was up-regulated in mitogenically stimulated B-2S. In both naive B-1 cells and stimulated B-2 cells, annexin II and S100A6 formed Ca++-sensitive complexes. These results confirm that the emerging field of proteomics detects differentially expressed molecules independently of RNA screening methods. These results identify two proteins (annexin II and S100A6) that are unexpectedly differentially expressed in B-1 cells and, although members of larger families, may fulfill unique, subset-specific functions. These results also validate 1D GE/LC-MS/MS as a reliable screening tool in identifying final protein product expression differences between B-1P and B-2S.