Human B cells promote T-cell plasticity to optimize antibody response by inducing coexpression of TH1/TFH signatures

Background: B cells mediate humoral immunity against pathogens but also direct CD4(+) T-cell responses. Recent plasticity studies in mice have challenged the concept of strict fate commitment during CD4(+) T-cell differentiation into distinct subsets. Objective: We sought to elucidate the contribution of human antigen-primed B cells in CD4(+) T-cell responses that support humoral immunity. Methods: CD4(+) T-cell differentiation by primary human B cells was investigated in in vitro cocultures by using tetanus toxoid and Salmonella species as antigen models. T-cell differentiation was assessed by using intracellular cytokines and subset-specific transcription factors and markers. IgM and IgG formation was analyzed by means of ELISA. Results: Human B cells, but not dendritic cells, induce prominent and stable coexpression of T(H)1 and follicular helper T (T-FH) cell characteristics during priming and on antigen recall. T(H)1/T-FH cells coexpress the T(H)1 and T-FH effector cytokines IFN-gamma and IL-21 and the T-FH marker CXCR5, demonstrating that the coexpressed T(H)1 and T-FH subset-specifying transcription factors T-box transcription factor (T-bet) and B cell lymphoma 6 are both functionally active. B cell-derived IL-6 and IL-12 controlled respective expression of IL-21 and IFN-gamma, with IL-21 being key for humoral immunity. Conclusion: Human B cells exploit CD4(+) T-cell plasticity to create flexibility in the effector T-cell response. Induction of a T-cell subset coexpressing IL-21 and IFN-gamma might combine IL-21-mediated T-cell aid for antibody production while maintaining T(H)1 cytokine expression to support other cellular immune defenses.