iNKT-cell help to B cells: a cooperative job between innate and adaptive immune responses.

T-cell help to B lymphocytes is one of the most important events in adaptive immune responses in health and disease. It is generally delivered by cognate CD4(+) T follicular helper (T(FH)) cells via both cell-to-cell contacts and soluble mediators, and it is essential for both the clonal expansion of antibody (Ab)-secreting B cells and memory B-cell formation. CD1d-restricted invariant natural killer T (iNKT) cells are a subset of innate-like T lymphocytes that rapidly respond to stimulation with specific lipid antigens (Ags) that are derived from infectious pathogens or stressed host cells. Activated iNKT cells produce a wide range of cytokines and upregulate costimulatory molecules that can promote activation of dendritic cells (DCs), natural killer (NK) cells, and T cells. A decade ago, we discovered that iNKT cells can help B cells to proliferate and to produce IgG Abs in vitro and in vivo. This adjuvant-like function of Ag-activated iNKT cells provides a flexible set of helper mechanisms that expand the current paradigm of T-cell-B-cell interaction and highlights the potential of iNKT-cell targeting vaccine formulations.