Cutting edge: Critical role of inducible costimulator in germinal center reactions

Inducible costimulator (ICOS) is a new member of the CD28/CTLA-4 family tha t is expressed on activated and germinal center (GC) T cells. Recently, we reported that ICOS-deficient mice exhibited profound defects in T cell acti vation and effector function. Ab responses in a T-dependent primary reactio n and in a murine asthma model were also diminished. In the current study, we investigate the mechanism by which ICOS regulates humoral immunity and e xamine B cell GC reactions in the absence of ICOS, We found that ICOS-/- mi ce, when immunized with SRBC, had smaller GCs. Furthermore, IgG1 class swit ching in the GCs was impaired, Remarkably, GC formation in response to a se condary recall challenge was completely absent in ICOS knockout mice. These data establish a critical role of ICOS in regulation of humoral immunity.