Arenavirus vectors for massive tumor self-antigen-specific CD8 T cell attack

Therapeutic vaccination regimens inducing high frequencies of clinically effective tumor-targeting CD8 T cells represent an unmet need in cancer immunotherapy.

We exploit cutting-edge viral reverse genetics to harness the immunostimulatory properties of the arenaviruses Pichinde virus and lymphocytic choriomeningitis virus for therapeutic cancer vaccination. When administering to mice life-replicating vector formats of these two viruses, both delivering the same cancer testis self-antigen in a heterologous prime-boost regimen, tumor-specific CD8 T cell responses reached up to 50% of the circulating CD8 T cell pool. This massive CD8 T cell attack eliminated established solid tumors in a significant proportion of animals, accompanied by complete protection against tumor re-challenge. The magnitude of CD8 T cell responses was driven by alarmin signals and depended on the combined use of two genealogically distantly related arenaviruses. Combinations of closely related vectors or the repeated administration of the same vector were not inhibited by vector-neutralizing antibodies, but CD8 T cell immunodominance hierarchies favored vector backbone-targeted responses at the expense of self-reactive T cell specificities.

These findings establish a powerful arenavirus-based tumor immunotherapy regimen exploiting key innate signaling pathways and reshuffling immunodominance hierarchies to break tumor self-directed tolerance

Arenavirus-based vector platform for massive tumor self-antigen-specific CD8 T cell immunity

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Background: The induction of powerful CD8+ T cell immunity to tumor associated self-antigens (TAAs) represents a critical yet challenging goal. Here we report on the development of an arenavirus-based delivery platform meeting this challenge. Previously we have shown that genetically engineered replication-attenuated lymphocytic choriomeningitis virus (LCMV) vectors, TheraT(LCMV), induce strong TAA-specific CD8 T cell immunity, but these responses can not be substantially augmented upon TheraT(LCMV) readministration. Counter to expectations, vector-neutralizing antibodies were not accountable for limited homologous prime-boosting capacity. Instead, dominant viral backbone-reactive CD8+ T cells competed against subdominant TAA-specific responses, limiting their magnitude. Methods: Herein we engineered and characterized delivery systems based on the arenaviruses Mopeia, Candid#1 and Pichinde (TheraT(MOP), TheraT(CAND), TheraT(PIC)). Results: We demonstrate that heterologous TheraT(CAND) – TheraT(LCMV) and TheraT(PIC)-TheraT(LCMV) prime-boost substantially augment TAA-specific CD8 T cell responses by rendering them immunodominant. Accordingly, intravenous administration of mice triggered up to 50% TAA epitope-specific CD8+ T cells and cured established tumors. Conversely, TheraT(MOP) – TheraT(LCMV) prime-boost was poorly immunogenic owing to cross-reactive T cell epitopes in the respective viral backbones. Conclusions: These findings establish heterologous arenavirus prime-boost combinations as a powerful new modality in tumor immunotherapy and highlight CD8 T cell epitope dominance as a significant hurdle to overcome in the vectored delivery of TAAs.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

The carboxy-terminal domain of glycoprotein N of human cytomegalovirus is required for virion morphogenesis

Glycoproteins M and N (gM and gN, respectively) are among the few proteins that are conserved across the herpesvirus family. The function of the complex is largely unknown. Whereas deletion from most alphaherpesviruses has marginal effects on the replication of the respective viruses, both proteins are essential for replication of human cytomegalovirus (HCMV). We have constructed a series of mutants in gN to study the function of this protein. gN of HCMV is a type I glycoprotein containing a short carboxy-terminal domain of 14 amino acids, including two cysteine residues directly adjacent to the predicted transmembrane anchor at positions 125 and 126. Deletion of the entire carboxy-terminal domain as well as substitution with the corresponding region from alpha herpesviruses or mutations of both cysteine residues resulted in a replication-incompetent virus. Recombinant viruses containing point mutations of either cysteine residue could be generated. These viruses were profoundly defective for replication. Complex formation of the mutant gNs with gM and transport of the complex to the viral assembly compartment appeared unaltered compared to the wild type. However, in infected cells, large numbers of capsids accumulated in the cytoplasm that failed to acquire an envelope. Transiently expressed gN was shown to be modified by palmitic acid at both cysteine residues. In summary, our data suggest that the carboxy-terminal domain of gN plays a critical role in secondary envelopment of HCMV and that palmitoylation of gN appears to be essential for function in secondary envelopment of HCMV and virus replication.