Monocytoid B cells are distinct from splenic marginal zone cells and commonly derive from unmutated naive B cells and less frequently from postgerminal center B cells by polyclonal transformation.

Monocytoid B cells represent a morphologically conspicuous B-cell population that constantly occurs in Toxoplasma gondii-induced Piringer's lymphadenopathy. Although widely believed to be closely related to splenic marginal zone B cells, neither this relationship, nor the B-cell differentiation stage of monocytoid B cells, nor their cellular precursors have been established. We have therefore examined monocytoid B cells for their expression of B-cell differentiation markers and the Ig isotypes at the RNA and protein level as well as for rearranged Ig heavy chain (H) genes and somatic mutations within the variable (V) region. The results obtained were compared with the corresponding features of other B-cell populations. The monocytoid B cells displayed immunophenotypical differences to all other B-cell populations. IgM and IgD expression was absent from most monocytoid B cells at the RNA and protein levels. Unrelated (polyclonal) Ig rearrangements were found in 85 of the 95 cells studied. Seventy-four percent of the rearranged VH genes were devoid of somatic mutations, whereas the remaining 26% carried a low number of somatic mutations. The majority of these showed no significant signs of antigen selection. This finding in conjunction with the predominantly unrelated Ig gene rearrangements indicates that most monocytoid B cells arise not by clonal proliferation but by transformation of polyclonal B cells. The B cells undergoing a monocytoid B-cell transformation are in the majority (74%) naive B cells, and only a minority are (26%) non-antigen-selected postgerminal center B cells. Thus, our data show that monocytoid B cells represent a distinct B-cell subpopulation.