T-bet-positive and IRTA1-positive monocytoid B cells differ from marginal zone B cells and epithelial-associated B cells in their antigen profile and topographical distribution.

BACKGROUND AND OBJECTIVES It remains controversial whether splenic and nodal marginal zone B cells, monocytoid -, and epithelial-associated B cells represent separate B-cell subsets or just variants of the same population. To clarify this issue we studied the antigen profile and topographical distribution of these cell types. DESIGN AND METHODS We studied samples of toxoplasmic lymphadenopathy, lymph nodes with a developed marginal zone, and hyperplastic palatine tonsils. Deparaffinized sections were subjected to antigen-retrieval pre-treatment then incubated with appropriate antibodies. The bound antibodies were made visible using the alkaline phosphatase anti-alkaline phosphatase method with FastRed as the chromogen. RESULTS We found that i) nodal marginal zone B cells have a similar immunophenotype and topographical distribution to their splenic counterparts, ii) monocytoid B cells differ in their antigen profile (presence of T-bet, IRTA1, CD75, CD45RA, absence of BCL-2 , CD21, CD27) from splenic and nodal marginal zone B cells and more closely resemble epithelial-associated B cells (presence of IRTA-1, CD45RA, partial expression of T-bet, CD75, absence of CD21, CD27). Monocytoid B cells were mostly not found in nodal marginal zones when a marginal zone was developed, but were seen in areas adjacent to marginal zones and occasionally in germinal centers. INTERPRETATION AND CONCLUSIONS Collectively, our results indicate that monocytoid B cells represent a distinct differentiated B-cell subset, and provide a solid basis for isolation and functional investigations of the cell types studied, and for revising the hitherto inadequate classification of nodal marginal zone lymphomas.