CD28-deficient mice are vulnerable to mouse papillomavirus MmuPV1 infection of the skin and mucosae.

CD28 is a co-stimulatory molecule expressed on the surface of T cells. To date, three individuals with germline CD28 deficiency have been reported to develop recalcitrant, HPV-driven warts: one exhibited persistent lesions, another experienced disease resolution, and the third developed a chronic "tree-man" phenotype. In mice, we confirmed that CD28-knockout (CD28ko) animals on the C57BL/6 (B6) background are susceptible to cutaneous infection with mouse papillomavirus (MmuPV1); however, their skin warts regressed spontaneously approximately five weeks post-infection. Furthermore, we demonstrate that CD28ko mice are vulnerable to MmuPV1 infection at HPV-relevant mucosal sites, including the most HPV prevalent sites: anogenital tract and oral cavity. Virions recovered from vaginal lavage were infectious but could be neutralized by the neutralizing monoclonal antibody MPV.A4. Viral clearance at mucosal sites was delayed in CD28ko mice, persisting for up to six weeks in the lower genital tract. Blocking the CD28 ligands CD80 and CD86 in B6 mice reproduced the CD28ko phenotype following MmuPV1 infection and markedly reduced CD28 expression, implicating the CD28-CD80/CD86 axis in delayed viral clearance. Infected CD28ko mice showed a reduction in both CD4+ and CD8+ T cell population in the spleen compared to infected B6 mice, but an increase in CD11c+/F4-80+ cells, particularly the plasmacytoid dendritic cell (pDCs, SiglecH⁺) subset. Additionally, CD28ko mice exhibited delayed recruitment of activated CD4+ T cells to infected tissues. Accumulation of MmuPV1 E6/90-99-specific, tetramer-positive CD8+ cytotoxic T lymphocytes (CTLs) was slower in CD28ko than in B6 mice; these CTLs remained FoxP3 negative but displayed reduced efficacy in both in vitro killing and antiviral cytokine assays. Adoptive transfer of CTLs from either B6 or CD28ko mice into MmuPV1-infected Rag1ko mice induced viral clearance at mucosal (oral) sites, whereas B6-derived CTLs achieved more complete regression of cutaneous (tail) lesions. Collectively, these findings indicate that CD28 deficiency delays but does not prevent the clearance of papillomavirus infections at both cutaneous and mucosal sites in mice.