Human Tfh cells proliferate in response to nivolumab in mice with a humanized immune system. (A–F) BRGST HIS mice were treated i.v. with 250 µg nivolumab or with PBS and analyzed 3 days later. (A) Experimental setup. (B) Lymph node reconstitution with human cells was confirmed by HLA-A, B, C staining of live cells. (C and D) Representative FACS dot plots (C) and quantification (D) of CD4⁺ BCL6⁺ CXCR5⁺ human T cells in mice treated with nivolumab or PBS. (E and F) Representative FACS dot plots (E) and quantification (F) of Ki67⁺ cells among CD4⁺ BCL6⁺ CXCR5⁺ human T cells. Compiled from three independent experiments with a total of 7–13 mice per group and generated from six different human donors. Statistical analyses were performed using t test (B, D, and F). *, P < 0.05.