Expression of the monocarboxylate transporter MCT1 is required for virus-specific mouse CD8+ T cell memory development.

Lactate-proton symporter monocarboxylate transporter 1 (MCT1) facilitates lactic acid export from T cells. Here, we report that MCT1 is mandatory for the development of virus-specific CD8+ T cell memory. MCT1-deficient T cells were exposed to acute pneumovirus (pneumonia virus of mice, PVM) or persistent γ-herpesvirus (Murid herpesvirus 4, MuHV-4) infection. MCT1 was required for the expansion of virus-specific CD8+ T cells and the control of virus replication in the acute phase of infection. This situation prevented the subsequent development of virus-specific T cell memory, a necessary step in containing virus reactivation during γ-herpesvirus latency. Instead, persistent active infection drove virus-specific CD8+ T cells toward functional exhaustion, a phenotype typically seen in chronic viral infections. Mechanistically, MCT1 deficiency sequentially impaired lactic acid efflux from activated CD8+ T cells, caused an intracellular acidification inhibiting glycolysis, disrupted nucleotide synthesis in the upstream pentose phosphate pathway, and halted cell proliferation which, ultimately, promoted functional CD8+ T cell exhaustion instead of memory development. Taken together, our data demonstrate that MCT1 expression is mandatory for inducing T cell memory and controlling viral infection by CD8+ T cells.

Functional Phenotyping of Lung Mouse CD4+ T Cells Using Multiparametric Flow Cytometry Analysis.

Gammaherpesviruses such as Epstein-Barr virus (EBV) are major modulators of the immune responses of their hosts. In the related study (PMID: 35857578), we investigated the role for Ly6Chi monocytes in shaping the function of effector CD4+ T cells in the context of a murine gammaherpesvirus infection (Murid gammaherpesvirus 4) as a model of human EBV. In order to unravel the polyfunctional properties of CD4+ T-cell subsets, we used multiparametric flow cytometry to perform intracellular staining on lung cells. As such, we have developed herein an intracellular staining workflow to identify on the same samples the cytotoxic and/or regulatory properties of CD4+ lymphocytes at the single-cell level. Briefly, following perfusion, collection, digestion, and filtration of the lung to obtain a single-cell suspension, lung cells were cultured for 4 h with protein transport inhibitors and specific stimulation media to accumulate cytokines of interest and/or cytotoxic granules. After multicolor surface labeling, fixation, and mild permeabilization, lung cells were stained for intracytoplasmic antigens and analyzed with a Fortessa 4-laser cytometer. This method of quantifying cytotoxic mediators as well as pro- or anti-inflammatory cytokines by flow cytometry has allowed us to decipher at high resolution the functional heterogeneity of lung CD4+ T cells recruited after a viral infection. Therefore, this analysis provided a better understanding of the importance of CD4+ T-cell regulation to prevent the development of virus-induced immunopathologies in the lung. Key features • High-resolution profiling of the functional properties of lung-infiltrating CD4+ T cells after viral infection using conventional multiparametric flow cytometry. • Detailed protocol for mouse lung dissection, preparation of single-cell suspension, and setup of multicolor surface/intracellular staining. • Summary of optimal ex vivo restimulation conditions for investigating the functional polarization and cytokine production of lung-infiltrating CD4+ T cells. • Comprehensive compilation of necessary biological and technical controls to ensure reliable data analysis and interpretation.

Dampening type 2 properties of group 2 innate lymphoid cells by a gammaherpesvirus infection reprograms alveolar macrophages.

Immunological dysregulation in asthma is associated with changes in exposure to microorganisms early in life. Gammaherpesviruses (γHVs), such as Epstein-Barr virus, are widespread human viruses that establish lifelong infection and profoundly shape host immunity. Using murid herpesvirus 4 (MuHV-4), a mouse γHV, we show that after infection, lung-resident and recruited group 2 innate lymphoid cells (ILC2s) exhibit a reduced ability to expand and produce type 2 cytokines in response to house dust mites, thereby contributing to protection against asthma. In contrast, MuHV-4 infection triggers GM-CSF production by those lung ILC2s, which orders the differentiation of monocytes (Mos) into alveolar macrophages (AMs) without promoting their type 2 functions. In the context of γHV infection, ILC2s are therefore essential cells within the pulmonary niche that imprint the tissue-specific identity of Mo-derived AMs and shape their function well beyond the initial acute infection.

Ly6Chi monocytes balance regulatory and cytotoxic CD4 T cell responses to control virus-induced immunopathology.

Gammaherpesviruses (γHVs) have coevolved with their host, leading to a remarkably high infection prevalence and establishment of latency. The lifelong persistence of γHVs in hosts appears to broadly shape host immunity, and we show here that pulmonary infection with Murid herpesvirus 4 (MuHV-4), a mouse γHV, drives the recruitment of Ly6Chi monocytes (MOs) into the airway, thereby modulating the host immune response. The absence of Ly6Chi MOs is associated with severe virus-induced immunopathology and the systemic release of inflammatory mediators. Mechanistically, MuHV-4-imprinted MOs recruit CD4 T cells to the airways and trigger immunosuppressive signaling pathways through the PD-L1/PD-1 axis, thereby dampening the deleterious activation of cytotoxic CD4 T cells. These results uncover a role for Ly6Chi MOs in modulating CD4 T cell functions and reveal pathways that could be targeted therapeutically to reduce detrimental immunopathological responses associated with respiratory viral infections.

A gammaherpesvirus licenses CD8 T cells to protect the host from pneumovirus-induced immunopathologies.

Human respiratory syncytial virus (RSV) is a pneumovirus that causes severe infections in infants worldwide. Despite intensive research, safe and effective vaccines against RSV have remained elusive. The main reason is that RSV infection of children previously immunized with formalin-inactivated-RSV vaccines has been associated with exacerbated pathology, a phenomenon called RSV vaccine-enhanced respiratory disease. In parallel, despite the high RSV prevalence, only a minor proportion of children develop severe diseases. Interestingly, variation in the immune responses against RSV or following RSV vaccination could be linked with differences of exposure to microbes during childhood. Gammaherpesviruses (γHVs), such as the Epstein-Barr virus, are persistent viruses that deeply influence the immune system of their host and could therefore affect the development of pneumovirus-induced immunopathologies for the long term. Here, we showed that a previous ɣHV infection protects against both pneumovirus vaccine-enhanced disease and pneumovirus primary infection and that CD8 T cells are essential for this protection. These observations shed a new light on the understanding of pneumovirus-induced diseases and open new perspectives for the development of vaccine strategies.