CCR2 Signaling Restricts SARS-CoV-2 Infection.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a historic pandemic of respiratory disease (coronavirus disease 2019 [COVID-19]), and current evidence suggests that severe disease is associated with dysregulated immunity within the respiratory tract. However, the innate immune mechanisms that mediate protection during COVID-19 are not well defined. Here, we characterize a mouse model of SARS-CoV-2 infection and find that early CCR2 signaling restricts the viral burden in the lung. We find that a recently developed mouse-adapted SARS-CoV-2 (MA-SARS-CoV-2) strain as well as the emerging B.1.351 variant trigger an inflammatory response in the lung characterized by the expression of proinflammatory cytokines and interferon-stimulated genes. Using intravital antibody labeling, we demonstrate that MA-SARS-CoV-2 infection leads to increases in circulating monocytes and an influx of CD45+ cells into the lung parenchyma that is dominated by monocyte-derived cells. Single-cell RNA sequencing (scRNA-Seq) analysis of lung homogenates identified a hyperinflammatory monocyte profile. We utilize this model to demonstrate that mechanistically, CCR2 signaling promotes the infiltration of classical monocytes into the lung and the expansion of monocyte-derived cells. Parenchymal monocyte-derived cells appear to play a protective role against MA-SARS-CoV-2, as mice lacking CCR2 showed higher viral loads in the lungs, increased lung viral dissemination, and elevated inflammatory cytokine responses. These studies have identified a potential CCR2-monocyte axis that is critical for promoting viral control and restricting inflammation within the respiratory tract during SARS-CoV-2 infection. IMPORTANCE SARS-CoV-2 has caused a historic pandemic of respiratory disease (COVID-19), and current evidence suggests that severe disease is associated with dysregulated immunity within the respiratory tract. However, the innate immune mechanisms that mediate protection during COVID-19 are not well defined. Here, we characterize a mouse model of SARS-CoV-2 infection and find that early CCR2-dependent infiltration of monocytes restricts the viral burden in the lung. We find that SARS-CoV-2 triggers an inflammatory response in the lung characterized by the expression of proinflammatory cytokines and interferon-stimulated genes. Using RNA sequencing and flow cytometry approaches, we demonstrate that SARS-CoV-2 infection leads to increases in circulating monocytes and an influx of CD45+ cells into the lung parenchyma that is dominated by monocyte-derived cells. Mechanistically, CCR2 signaling promoted the infiltration of classical monocytes into the lung and the expansion of monocyte-derived cells. Parenchymal monocyte-derived cells appear to play a protective role against MA-SARS-CoV-2, as mice lacking CCR2 showed higher viral loads in the lungs, increased lung viral dissemination, and elevated inflammatory cytokine responses. These studies have identified that the CCR2 pathway is critical for promoting viral control and restricting inflammation within the respiratory tract during SARS-CoV-2 infection.

CCR2 Regulates Vaccine-Induced Mucosal T-Cell Memory to Influenza A Virus.

Elicitation of lung tissue-resident memory CD8 T cells (TRMs) is a goal of T cell-based vaccines against respiratory viral pathogens, such as influenza A virus (IAV). C-C chemokine receptor type 2 (CCR2)-dependent monocyte trafficking plays an essential role in the establishment of CD8 TRMs in lungs of IAV-infected mice. Here, we used a combination adjuvant-based subunit vaccine strategy that evokes multifaceted (TC1/TC17/TH1/TH17) IAV nucleoprotein-specific lung TRMs to determine whether CCR2 and monocyte infiltration are essential for vaccine-induced TRM development and protective immunity to IAV in lungs. Following intranasal vaccination, neutrophils, monocytes, conventional dendritic cells (DCs), and monocyte-derived dendritic cells internalized and processed vaccine antigen in lungs. We found that basic leucine zipper ATF-like transcription factor 3 (BATF3)-dependent DCs were essential for eliciting T cell responses, but CCR2 deficiency enhanced the differentiation of CD127hi, KLRG-1lo, OX40+ve CD62L+ve, and mucosally imprinted CD69+ve CD103+ve effector and memory CD8 T cells in lungs and airways of vaccinated mice. Mechanistically, increased development of lung TRMs induced by CCR2 deficiency was linked to dampened expression of T-bet but not altered TCF-1 levels or T cell receptor signaling in CD8 T cells. T1/T17 functional programming, parenchymal localization of CD8/CD4 effector and memory T cells, recall T cell responses, and protective immunity to a lethal IAV infection were unaffected in CCR2-deficient mice. Taken together, we identified a negative regulatory role for CCR2 and monocyte trafficking in mucosal imprinting and differentiation of vaccine-induced TRMs. Mechanistic insights from this study may aid the development of T-cell-based vaccines against respiratory viral pathogens, including IAV and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). IMPORTANCE While antibody-based immunity to influenza A virus (IAV) is type and subtype specific, lung- and airway-resident memory T cells that recognize conserved epitopes in the internal viral proteins are known to provide heterosubtypic immunity. Hence, broadly protective IAV vaccines need to elicit robust T cell memory in the respiratory tract. We have developed a combination adjuvant-based IAV nucleoprotein vaccine that elicits strong CD4 and CD8 T cell memory in lungs and protects against H1N1 and H5N1 strains of IAV. In this study, we examined the mechanisms that control vaccine-induced protective memory T cells in the respiratory tract. We found that trafficking of monocytes into lungs might limit the development of antiviral lung-resident memory T cells following intranasal vaccination. These findings suggest that strategies that limit monocyte infiltration can potentiate vaccine-induced frontline T-cell immunity to respiratory viruses, such as IAV and SARS-CoV-2.