In vivo detection of antigen-specific CD8+ T cells by immuno-positron emission tomography.

The immune system's ability to recognize peptides on major histocompatibility molecules contributes to the eradication of cancers and pathogens. Tracking these responses in vivo could help evaluate the efficacy of immune interventions and improve mechanistic understanding of immune responses. For this purpose, we employ synTacs, which are dimeric major histocompatibility molecule scaffolds of defined composition. SynTacs, when labeled with positron-emitting isotopes, can noninvasively image antigen-specific CD8+ T cells in vivo. Using radiolabeled synTacs loaded with the appropriate peptides, we imaged human papillomavirus-specific CD8+ T cells by positron emission tomography in mice bearing human papillomavirus-positive tumors, as well as influenza A virus-specific CD8+ T cells in the lungs of influenza A virus-infected mice. It is thus possible to visualize antigen-specific CD8+ T-cell populations in vivo, which may serve prognostic and diagnostic roles.

Nanobody-Antigen Conjugates Elicit HPV-Specific Antitumor Immune Responses.

High-risk human papillomavirus-associated cancers express viral oncoproteins (e.g., E6 and E7) that induce and maintain the malignant phenotype. The viral origin of these proteins makes them attractive targets for development of a therapeutic vaccine. Camelid-derived single-domain antibody fragments (nanobodies or VHHs) that recognize cell surface proteins on antigen-presenting cells (APC) can serve as targeted delivery vehicles for antigens attached to them. Such VHHs were shown to induce CD4+ and CD8+ T-cell responses against model antigens conjugated to them via sortase, but antitumor responses had not yet been investigated. Here, we tested the ability of an anti-CD11b VHH (VHHCD11b) to target APCs and serve as the basis for a therapeutic vaccine to induce CD8+ T-cell responses against HPV+ tumors. Mice immunized with VHHCD11b conjugated to an H-2Db-restricted immunodominant E7 epitope (E749-57) had more E7-specific CD8+ T cells compared with those immunized with E749-57 peptide alone. These CD8+ T cells acted prophylactically and conferred protection against a subsequent challenge with HPV E7-expressing tumor cells. In a therapeutic setting, VHHCD11b-E749-57 vaccination resulted in greater numbers of CD8+ tumor-infiltrating lymphocytes compared with mice receiving E749-57 peptide alone in HPV+ tumor-bearing mice, as measured by in vivo noninvasive VHH-based immune-positron emission tomography (immunoPET), which correlated with tumor regression and survival outcome. Together, these results demonstrate that VHHs can serve as a therapeutic cancer vaccine platform for HPV-induced cancers. Cancer Immunol Res; 6(7); 870-80. ©2018 AACR.