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MCMV expressing high affinity NKG2D ligand MULT-1: a model vaccine for congenital cmv infection

Introduction: Cytomegaloviruses (CMVs) encode numerous non-essential immunoevasive genes which can be deleted or replaced by any other gene without affecting viral growth or fitness in vitro. Deletion of certain viral immunoevasins attenuates the virus in vivo, thus enabling the generation of viruses with altered virulence, which can be used as vaccine vectors. We have previously shown that murine CMV (MCMV) expressing RAE-1γ (one of the cellular ligands for the NKG2D receptor) is highly attenuated in vivo but retains the capacity to induce strong adaptive immune response to viral and vectored antigens. MULT-1 is a NKG2D ligand with highest affinity for the receptor. We constructed recombinant MCMV expressing high affinity ligand MULT-1 (MULT-1MCMV) and compared it with RAE-1γMCMV and MCMV to further explore vaccine properties of such virus. Materials and Methods: MULT-1MCMV was constructed by insertion of gene for NKG2D ligand MULT-1 in the place of its viral regulator m145. Viral spread in vivo was determined by infection of adult and newborn mice and assessment of viral titer in their organs. Capacity to induce CD8 T cell response was determined in vivo by evaluating antigen-specific CD8 T cells in infected animals and in vitro by CD8 T cell antigen presentation assay. Humoral response was determined by analysis of virus-specific antibodies in the sera of infected mice and offspring of immunized mothers. Results: We demonstrate that a recombinant MCMV expressing high affinity NKG2D ligand MULT-1 is highly attenuated in vivo by NK cells in a NKG2D-dependent. MULT-1MCMV is capable of inducing antigen-specific CD8 T cells and humoral response. In newborn mice, MULT-1MCMV was controlled more rapidly compared to RAE- 1γMCMV and failed to reach the brain of infected newborn mice. As a result, newborn mice infected with this virus were devoid of brain inflammation and microglia polarization. Conclusion: Our study showed that expression of high affinity NKG2D ligand MULT-1 in MCMV mediates its strong immune control while retaining capacity to induce robust cellular and humoral immunity. Efficient virus control and absence of immunopathology of MULT-1MCMV was particularly pronounced upon infection of neonatal mice. These findings demonstrate that CMVbased vaccine vectors expressing high affinity NKG2D ligand are promising vaccine candidates, which could be used in immunologically immature and immunocompromised patients.

MCMV, NK cells, CD8 T cells, NKG2D, congenital infection

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