CTLA4 and influenza: We have previously shown that a pronounced induction of antigen-specific T-cell levels after infection with SARS-CoV-220, with varicella zoster virus21, or after influenza vaccination22 is paralleled by an upregulation of CTLA-4 on specific T cells which may serve to counteract excessive T-cell proliferation and/or T-cell mediated immunopathology.