Inducing a human atypical memory B cell phenotype <i>in vitro</i>

Abstract Chronic human infectious diseases such as malaria are associated with expansion of an unusual population of memory B cells (MBCs) referred to as atypical MBCs. In malaria, atypical MBCs are CD21−, CD27−, highly express the transcription factor, T-bet, and also express a variety of inhibitory receptors, including FcRL5 and Siglec6. These atypical MBCs signal poorly through their B cell receptors (BCRs) and exhibit poor B cell function. We concluded from these studies that atypical MBCs contribute to the slow, inefficient acquisition of immunity to malaria in African children. However, the mechanisms by which atypical MBCs are induced during chronic infectious diseases are not known. Acute febrile malaria in African children is correlated with expansion of less-functional TH1-type T follicular helper T cells which secrete IFN-γ and are impaired in their helper function, suggesting that they may contribute to the generation of atypical MBCs. We investigated the conditions under which human naïve B cells were induced to express an atypical MBC phenotype in vitro. Studying human tonsillar B cells we showed that BCR cross linking by anti-Ig in the presence of IFN-γ and CpG, induced high levels of T-bet and FcRL5. Moreover, expression of these markers was further enhanced by the persistent engagement of B cells with membrane bound anti-Ig. We also found that atypical MBCs induced in vitro by persistent antigen, CpG and IFN-γ showed severely reduced early BCR signaling when re-challenged with soluble antigen, characteristic of atypical MBCs. Taken together these results suggest that atypical MBCs may be the product of persistent antigen engagement by B cells in the highly inflammatory environments that develop during chronic malaria exposure.