As chronic antigenic stimulation from infection and autoimmunity is a feature of primary antibody deficiency (PAD), analysis of affected patients could yield insights into T-cell differentiation and explain how environmental exposures modify clinical phenotypes conferred by single-gene defects. CD57 marks dysfunctional T cells that have differentiated after antigenic stimulation. Indeed, while circulating CD57+CD4+T cells are normally rare, we found that they are increased in patients with PAD and markedly increased withCTLA4haploinsufficiency or blockade. We performed single-cell RNA-seq analysis of matched CD57+CD4+T cells from blood and tonsil samples. Circulating CD57+CD4+T cells (CD4cyt) exhibited a cytotoxic transcriptome similar to that of CD8+effector cells, could kill B cells, and inhibited B-cell responses. CTLA4 restrained the formation of CD4cyt. While CD57 also marked an abundant subset of follicular helper T cells, which is consistent with their antigen-driven differentiation, this subset had a pre-exhaustion transcriptomic signature marked byTCF7,TOX, andID3expression and constitutive expression of CTLA4 and did not become cytotoxic even after CTLA4 inhibition. Thus, CD57+CD4+T-cell cytotoxicity and exhaustion phenotypes are compartmentalised between blood and germinal centers. CTLA4 is a key modifier of CD4+T-cell cytotoxicity, and the pathological CD4cyt phenotype is accentuated by infection.