Table 1.

Functional factors and roles in B-cell activation of Tfh and Tfr cells.

Tfh cell subsets	Functional factors	Functions	References
GC Tfh	IL-21, IL-4, CD40L	Help for B-cell maturation, high-affinity antibody production, antibody class switching, and memory B-cell development.	(4, 8, 10, 40–43)
cTfh
cTfh1	IL-21, IFN-γ, IL-10	Help for memory B-cell differentiation into plasma cells.	(8, 10, 19, 20, 44–47)
cTfh2	IL-21, IL-4, IL-5, IL-13	Help for IgM, IgG, IgA, and IgE secretion.
cTfh17	IL-21, IL-17, IL-22	Help for IgM, IgG, and IgA secretion.
cTfh17.1	IL-21, IL-17, IL-22, IFN-γ	Help for IgM, IgG, and IgA secretion.
Tph	IL-21, SLAMF5, CXCL13	Help for B-cell function with a migratory program targeting inflamed tissues and recruitment of Tfh and B cells to inflamed tissues.	(26, 48)
GC Tfr	CTLA-4, IL-10, TGF-β, granzyme B	Strong suppression on Tfh-cell and B-cell responses in GC.	(13, 15, 23)
cTfr	CTLA-4, IL-10, TGF-β, granzyme B	Less suppression of Tfh-cell and B-cell responses in GC.	(6, 49–52)

GC, germinal centers; Tfh, T follicular helper; cTfh, circulating Tfh; Tph, T peripheral helper; Tfr, T follicular regulatory; IFN, interferon; IL, interleukin; TGF, transforming growth factor; SLAMF, signaling lymphocyte activation molecule family; Ig, immunoglobulin; CTLA-4, cytotoxic T-lymphocyte antigen 4.