In this article we report that the M2 protein encoded by the vaccinia virus is secreted as a homo-oligomer by infected cells and binds two central costimulation molecules, CD80 (B7-1) and CD86 (B7-2). These interactions block the ligation of the two B7 proteins to both soluble CD28 and soluble cytotoxic T-lymphocyte associated protein 4 (CTLA4) but favor the binding of soluble PD-L1 to soluble CD80. M2L gene orthologues are found in several other poxviruses, and the B7-CD28/CTLA4 blocking activity has been identified for several culture supernatants of orthopoxvirus-infected cells and for a recombinant myxoma virus M2 protein homolog (i.e., Gp120-like protein, or Gp120LP). Overall, these data indicate that the M2 poxvirus family of proteins may be involved in immunosuppressive activities broader than the NF-κB inhibition already reported (R. Gedey, X. L. Jin, O. Hinthong, and J. L. Shisler, J Virol 80:8676–8685, 2006, https://doi.org/10.1128/JVI.00935-06). A Copenhagen vaccinia virus with a deletion of the nonessential M2L locus was generated and compared with its parental virus. This M2L-deleted vaccinia virus, unlike the parental virus, does not generate interference with the B7-CD28/CTLA4/PD-L1 interactions. Moreover, this deletion did not affect any key features of the virus (in vitro replication, oncolytic activities in vitro and in vivo, and intratumoral expression of a transgene in an immunocompetent murine model). Altogether, these first results suggest that the M2 protein has the potential to be used as a new immunosuppressive biotherapeutic and that the M2L-deleted vaccinia virus represents an attractive new oncolytic platform with an improved immunological profile. IMPORTANCE The vaccinia virus harbors in its genome several genes dedicated to the inhibition of the host immune response. Among them, M2L was reported to inhibit the intracellular NF-κB pathway. We report here several new putative immunosuppressive activities of M2 protein. M2 protein is secreted and binds cornerstone costimulatory molecules (CD80/CD86). M2 binding to CD80/CD86 blocks their interaction with soluble CD28/CTLA4 but also favors the soluble PD-L1-CD80 association. These findings open the way for new investigations deciphering the immune system effects of soluble M2 protein. Moreover, a vaccinia virus with a deletion of its M2L has been generated and characterized as a new oncolytic platform. The replication and oncolytic activities of the M2L-deleted vaccinia virus are indistinguishable from those of the parental virus. More investigations are needed to characterize in detail the immune response triggered against both the tumor and the virus by this M2-defective vaccinia virus.

Background: Single domain antibodies (sdAbs) isolated after immunization of camelids are particularly attractive formats for their high modularity and small size allowing a better diffusion into tumors. However, the short in vivo half-life of sdAbs, related to the lack of a Fc domain, limits their clinical application. By replicating specifically into tumor cells, the oncolytic vaccinia virus (VACV) is an optimal vector to deliver and maintain high intra-tumoral concentrations of therapeutic sdAb. Moreover, sdAb targeting immunological targets, such as PD-L1, may synergize the anti-tumoral activity of VACV. Randox and Transgene report the selection and characterization of a sdAb targeting the human PD-L1 and the design of optimal formats, including bispecific anti-PD-L1-CD40 ligand, for vectorization into VACV. Methods: Alpacas were immunized with human PD-L1 protein and sdAb coding sequences were isolated by PCR. Anti-PD-L1 sdAb binders were selected by phage display and sdAb blockers of PD-L1/PD-1 interaction were identified by ELISA. The ability of the selected sdAb to disrupt the PD-L1/PD-1 interaction was verified on transformed and primary cells. To fine-tune an optimal anti-PD-L1, several sdAb formats were designed and vectorized into VACV. The sdAb format exhibiting the best PD-L1/PD-1 blocking activity was selected by the screening of culture supernatants of several VACV-sdAb infected tumor cells. Finally, anti-PD-L1 sdAb-CD40L fusions were designed to generate a strong CD40 agonist active only in a PD-L1 positive environment. Results: SdAb clone 1A1 exhibited the best PD-L1/PD-1 blocking activity which remained unchanged after extensive humanization (latterly becoming named GS542). GS542 was vectorized in VACV as monomeric single chain homodimer, and fused to Fc domain or antibody hinge domain to foster dimerization together with full length IgG1 avelumab as anti-PD-L1 benchmark. All constructs were expressed by infected tumor cells. The single chain homodimer displayed the best PD-L1/PD-1 blocking activity, superior to that of avelumab. Moreover, GS542-CD40L fusions were designed to take advantage of the natural trimerization of CD40L to increase the avidity for PD-L1 while clustering CD40L at the surface of PD-L1+ cells to trans-activate the CD40 pathway. Evaluation of these GS542-CD40L fusions showed strong CD40 agonist activity depending on the presence of PD-L1+ cells making these constructs safer CD40 agonists. Conclusion: An anti-PD-L1 sdAb with a strong blocking activity was selected, humanized and evaluated under different VACV-vectorized formats. The single chain homodimeric sdAb expressed by VACV was identified as the best PD-L1/PD-1 blocking format. Furthermore, bispecific anti-PD-L1 sdAb-CD40L fusions that exhibited strong CD40 agonist activity was charactered within a PD-L1+ environment. Citation Format: Jean-Baptiste Marchand, Elodie Pintado, Marshall Dunlop, Christelle Remy, Patricia Kleinpeter, Shirley Shön, Fend Laetitia, Renée Brandely, Delphine Suhner, Eline Winter, Nathalie Silvestre, Claire Huguet, Peter Fitzgerald, Eric Quéméneur. Selection of an optimal anti-PD-L1 single domain antibody format for the vectorization into oncolytic vaccinia virus and the generation of bispecific immunomodulators [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1885.