Oncolytic measles vaccines encoding PD-1 and PD-L1 checkpoint blocking antibodies to increase tumor-specific T cell memory.

PD-1/PD-L1 checkpoint blockade has achieved unprecedented success in cancer immunotherapy. Nevertheless, many immune-excluded tumors are resistant to therapy. Combination with oncolytic virotherapy may overcome resistance by inducing acute inflammation, immune cell recruitment, and remodeling of the tumor immune environment. Here, we assessed the combination of oncolytic measles vaccine (MV) vectors and PD-1/PD-L1 blockade. In the MC38cea model of measles virus oncolysis, MV combined with anti-PD-1 and MV vectors encoding anti-PD-1 or anti-PD-L1 antibodies achieved modest survival benefits compared with control MV or vectors encoding the antibody constant regions only. Analyses of tumor samples and tumor-draining lymph nodes revealed slight increases in intratumoral T cell effector cytokines as well as a shift toward an effector memory phenotype in the T cell compartment. Importantly, increased IFN-γ recall responses were observed in tumor rechallenge experiments with mice in complete tumor remission after treatment with MV encoding anti-PD-1 or anti-PD-L1 compared with control MV. These results prompted us to generate MV encoding the clinically approved agents pembrolizumab and nivolumab. Previously, we have generated MV encoding atezolizumab. We demonstrated the functionality of the novel vectors in vitro. We envision these vectors as therapeutics that induce and support durable anti-tumor immune memory.

Virotherapy combined with anti-PD-1 transiently reshapes the tumor immune environment and induces anti-tumor immunity in a preclinical PDAC model.

Introduction: Pancreatic ductal adenocarcinoma (PDAC) is largely refractory to cancer immunotherapy with PD-1 immune checkpoint blockade (ICB). Oncolytic virotherapy has been shown to synergize with ICB. In this work, we investigated the combination of anti-PD-1 and oncolytic measles vaccine in an immunocompetent transplantable PDAC mouse model. Methods: We characterized tumor-infiltrating T cells by immunohistochemistry, flow cytometry and T cell receptor sequencing. Further, we performed gene expression profiling of tumor samples at baseline, after treatment, and when tumors progressed. Moreover, we analyzed systemic anti-tumor and anti-viral immunity. Results: Combination treatment significantly prolonged survival compared to monotherapies. Tumor-infiltrating immune cells were increased after virotherapy. Gene expression profiling revealed a unique, but transient signature of immune activation after combination treatment. However, systemic anti-tumor immunity was induced by virotherapy and remained detectable even when tumors progressed. Anti-PD-1 treatment did not impact anti-viral immunity. Discussion: Our results indicate that combined virotherapy and ICB induces anti-tumor immunity and reshapes the tumor immune environment. However, further refinement of this approach may be required to develop its full potential and achieve durable efficacy.

Mechanisms of measles virus oncolytic immunotherapy.

The study of measles virus (MeV) as a cancer immunotherapeutic was prompted by clinical observations of leukemia and lymphoma regressions in patients following measles virus infection in the 1970s and 1980s. Since then, numerous preclinical studies have confirmed the oncolytic activity of MeV vaccine strains as well as their potential to promote long-lasting tumor-specific immune responses. Early clinical data indicate that some of these effects may translate to the treatment of cancer patients. In this review, we provide a structured summary of current evidence for the anti-tumor immune activity of oncolytic MeV. We start with an overview of MeV oncolysis and MeV-induced immunogenic cell death. Next, we relate findings on MeV-mediated activation of antigen-presenting cells, T cell priming and effector mechanisms to the cancer immunity cycle. We discuss additional factors in the tumor microenvironment which are modulated by MeV treatment as well as the role of anti-viral immunity. Based on these findings, we highlight avenues for rational enhancement of oncolytic MeV immunotherapy by vector engineering. We further point to advantages and drawbacks of experimental models and propose areas warranting promising research. Lastly, we review the available immunomonitoring data from several Phase I clinical trials. While this review presents data for MeV, the concepts and principles introduced herein apply to other oncolytic viruses, providing a framework to assess novel cancer immunotherapies.

Measles Vaccines Designed for Enhanced CD8+ T Cell Activation.

Priming and activation of CD8+ T cell responses is crucial to achieve anti-viral and anti-tumor immunity. Live attenuated measles vaccine strains have been used successfully for immunization for decades and are currently investigated in trials of oncolytic virotherapy. The available reverse genetics systems allow for insertion of additional genes, including heterologous antigens. Here, we designed recombinant measles vaccine vectors for priming and activation of antigen-specific CD8+ T cells. For proof-of-concept, we used cytotoxic T lymphocyte (CTL) lines specific for the melanoma-associated differentiation antigen tyrosinase-related protein-2 (TRP-2), or the model antigen chicken ovalbumin (OVA), respectively. We generated recombinant measles vaccine vectors with TRP-2 and OVA epitope cassette variants for expression of the full-length antigen or the respective immunodominant CD8+ epitope, with additional variants mediating secretion or proteasomal degradation of the epitope. We show that these recombinant measles virus vectors mediate varying levels of MHC class I (MHC-I)-restricted epitope presentation, leading to activation of cognate CTLs, as indicated by secretion of interferon-gamma (IFNγ) in vitro. Importantly, the recombinant OVA vaccines also mediate priming of naïve OT-I CD8+ T cells by dendritic cells. While all vaccine variants can prime and activate cognate T cells, IFNγ release was enhanced using a secreted epitope variant and a variant with epitope strings targeted to the proteasome. The principles presented in this study will facilitate the design of recombinant vaccines to elicit CD8+ responses against pathogens and tumor antigens.

Clonal selection drives protective memory B cell responses in controlled human malaria infection.

Affinity maturation, the clonal selection and expansion of antigen-activated B cells expressing somatically mutated antibody variants that develop during T cell-dependent germinal center reactions, is considered pivotal for efficient development of protective B cell memory responses to infection and vaccination. Repeated antigen exposure promotes affinity maturation but each time also recruits antigen-reactive naïve B cells into the response. Here, we determined the relative impact of affinity maturation versus antigen-mediated clonal selection of naïve B cells to mount potent B cell memory responses in humans after repeated exposure to a complex pathogen, the malaria parasite Plasmodium falciparum (Pf). Using single-cell immunoglobulin (Ig) gene sequencing and production of recombinant monoclonal antibodies, we analyzed the origin, development, and quality of memory B cell responses to Pf circumsporozoite protein (PfCSP), the major sporozoite surface protein. We show that after repeated immunization of Pf-naïve volunteers with infectious Pf sporozoites (PfSPZ Challenge) under chloroquine prophylaxis (PfSPZ-CVac), the clonal selection of potent germline and memory B cell precursors against the central PfCSP NANP repeat outpaces affinity maturation because the majority of Ig gene mutations are affinity-neutral. Mathematical modeling explains how the efficiency of affinity maturation decreases strongly with antigen complexity. Thus, in the absence of long-term exposure, the frequency of antigen-reactive precursors and likelihood of their activation rather than affinity maturation will determine the quality of anti-PfCSP memory B cell responses. These findings have wide implications for the design of vaccination strategies to induce potent B cell memory responses against PfCSP and presumably other structurally complex antigens.