CD70 provides costimulation that enhances effector T cell differentiation upon binding of its receptor, CD27. During chronic immune activation, CD70 is constitutively expressed on activated immune cells, and this induces T cell-driven disruption of neutralizing Ab responses via an unknown mechanism. We used CD70-transgenic mice to investigate the effect of constitutive expression of CD70 on T cell-dependent B cell responses. CD70 induced up-regulation of the B cell follicle homing chemokine receptor CXCR5 on T cells, enabling not only CD4 but also CD8 T cells to infiltrate the B cell follicles. CD70-transgenic mice failed to develop productive germinal center formation and displayed impaired IgG Ab responses. Defective germinal center B cell differentiation was critically dependent on CD70-mediated CD27 signaling in T cells, and involved Fas-dependent impairment of germinal center B cell differentiation. Thus, CD70-driven costimulation enables T cells to terminate B cell responses, thereby compromising durable Ab production. Our findings imply that the CD70- and CD27-driven costimulatory axis may be involved in shutdown of B cell responses before clearance of Ag. Because CD70 is expressed constitutively in chronic viral infections such as HIV-1 infection, this mechanism may also contribute to defects in humoral immunity associated with this disease.