To further delineate immune function of FLRT3 in vivo, we performed a CDX tumor and graft-versus-host disease (GVHD) model with adoptively transferred human PBMCs and determined whether FLRT3 Fc administration regulated human T cell immune responses against both human tumor cells (allogeneic mismatch) and mouse tissues (xenogeneic mismatch). Here, FLRT3 is linked to graft versus host disease.