Development and function of protective and pathologic memory CD4 T cells

Immunological memory is one of the defining features of the adaptive immune system. As key orchestrators and mediators of immunity, CD4 T cells are central to the vast majority of adaptive immune responses. Generated following an immune response, memory CD4 T cells retain pertinent information about their activation environment enabling them to make rapid effector responses upon reactivation. These responses can either benefit the host by hastening the control of pathogens or cause damaging immunopathology. Here, we will discuss the diversity of the memory CD4 T cell pool, the signals that influence the transition of activated T cells into that pool, and highlight how activation requirements differ between naïve and memory CD4 T cells. A greater understanding of these factors has the potential to aid the design of more effective vaccines and to improve regulation of pathologic CD4 T cells, such as in the context of autoimmunity and allergy

The roles of resident, central and effector memory CD4 T cells in protective immunity following infection or vaccination

Immunological memory provides rapid protection to pathogens previously encountered through infection or vaccination. CD4 T cells play a central role in all adaptive immune responses. Vaccines must, therefore, activate CD4 T cells if they are to generate protective immunity. For many diseases, we do not have effective vaccines. These include HIV, tuberculosis and malaria, which are responsible for many millions of deaths each year across the globe. CD4 T cells play many different roles during the immune response coordinating the actions of many other cells. In order to harness the diverse protective effects of memory CD4 T cells we need to understand how memory CD4 T cells are generated and how they protect the host. Here we review recent findings on the location of different subsets of memory CD4 T cells that are found in peripheral tissues (tissue resident memory T cells) and in the circulation (central and effector memory T cells). We discuss the generation of these cells and the evidence that demonstrates how they provide immune protection in animal and human challenge models

Stromal‐immune cell cross talk fundamentally alters the lung microenvironment following tissue insult

Communication between stromal and immune cells is essential to maintain tissue homeostasis, mount an effective immune response, and promote tissue repair. This ‘crosstalk’ occurs in both the steady state and following a variety of insults, for example, in response to local injury, at sites of infection, or cancer. What do we mean by crosstalk between cells? Reciprocal activation and/or regulation occurs between immune and stromal cells, by direct cell contact and indirect mechanisms, including the release of soluble cytokines. Moving beyond cell to cell contact, and this review investigates the complexity of ‘cross‐space’ cellular communication. We highlight different examples of cellular communication by a variety of lung stromal and immune cells following tissue insults. This review examines how the ‘geography of the lung microenvironment’ is altered in various disease states, more specifically we investigate how this influences lung epithelial cells and fibroblasts via their communication with immune cells and each other

Antigen presenting cells: professionals, amateurs, and spectators in the 'long game' of lung immunity

The lung is frequently and repeatedly exposed to invading pathogens and thus requires constant immunosurveillance. Professional antigen presenting cells (APCs), including dendritic cells, engulf invading pathogens and present their peptides via major histocompatibility complexes (MHC) I and II, to CD8 or CD4 T cells. Epithelial cells and stromal cells (including fibroblasts) provide more than structural support, they are increasingly recognised as key players in the immune response, acting as non-professional APCs through interactions with antigen experienced T cells that migrate to the lung. The importance of the contributions of non-professional and professional APCs to T cell function in vivo, is currently unclear. This review summarises the roles of professional and non-professional APCs in lung immunity, at the steady state and following viral insult, with particular emphasis on their ability to interact with and influence T cells

Regulatory T cells control the dynamic and site-specific polarization of total CD4 T cells following Salmonella infection

FoxP3+ regulatory T cells (Tregs) control inflammation and maintain mucosal homeostasis, but their functions during infection are poorly understood. Th1, Th2, and Th17 cells can be identified by master transcription factors (TFs) T-bet, GATA3, and RORγT; Tregs also express these TFs. While T-bet+ Tregs can selectively suppress Th1 cells, it is unclear whether distinct Treg populations can alter Th bias. To address this, we used Salmonella enterica serotype Typhimurium to induce nonlethal colitis. Following infection, we observed an early colonic Th17 response within total CD4 T cells, followed by a Th1 bias. The early Th17 response, which contains both Salmonella-specific and non-Salmonella-specific cells, parallels an increase in T-bet+ Tregs. Later, Th1 cells and RORγT+ Tregs dominate. This reciprocal dynamic may indicate that Tregs selectively suppress Th cells, shaping the immune response. Treg depletion 1–2 days post-infection shifted the early Th17 response to a Th1 bias; however, Treg depletion 6–7 days post-infection abrogated the Th1 bias. Thus, Tregs are necessary for the early Th17 response, and for a maximal Th1 response later. These data show that Tregs shape the overall tissue CD4 T cell response and highlight the potential for subpopulations of Tregs to be used in targeted therapeutic approaches

Multifunctional cytokine production reveals functional superiority of memory CD4 T cells

T cell protective immunity is associated with multifunctional memory cells that produce several different cytokines. Currently, our understanding of when and how these cells are generated is limited. We have used an influenza virus mouse infection model to investigate whether the cytokine profile of memory T cells is reflective of primary responding cells or skewed towards a distinct profile. We found that, in comparison to primary cells, memory T cells tended to make multiple cytokines simultaneously. Analysis of the timings of release of cytokine by influenza virus‐specific T cells, demonstrated that primary responding CD4 T cells from lymphoid organs were unable to produce a sustained cytokine response. In contrast CD8 T cells, memory CD4 T cells, and primary responding CD4 T cells from the lung produced a sustained cytokine response throughout the restimulation period. Moreover, memory CD4 T cells were more resistant than primary responding CD4 T cells to inhibitors that suppress T cell receptor signalling. Together, these data suggest that memory CD4 T cells display superior cytokine responses compared to primary responding cells. These data are key to our ability to identify the cues that drive the generation of protective memory CD4 T cells following infection

The atypical chemokine receptor Ackr2 constrains NK cell migratory activity and promotes metastasis

Chemokines have been shown to be essential players in a range of cancer contexts. In this study, we demonstrate that mice deficient in the atypical chemokine receptor Ackr2 display impaired development of metastasis in vivo in both cell line and spontaneous models. Further analysis reveals that this relates to increased expression of the chemokine receptor CCR2, specifically by KLRG1+ NK cells from the Ackr2−/− mice. This leads to increased recruitment of KLRG1+ NK cells to CCL2-expressing tumors and enhanced tumor killing. Together, these data indicate that Ackr2 limits the expression of CCR2 on NK cells and restricts their tumoricidal activity. Our data have important implications for our understanding of the roles for chemokines in the metastatic process and highlight Ackr2 and CCR2 as potentially manipulable therapeutic targets in metastasis

Triphasic production of IFNγ by innate and adaptive lymphocytes following influenza A virus infection

Interferon gamma (IFNγ) is a potent antiviral cytokine that can be produced by many innate and adaptive immune cells during infection. Currently, our understanding of which cells produce IFNγ and where they are located at different stages of an infection is limited. We have used reporter mice to investigate in vivo expression of Ifnγ mRNA in the lung and secondary lymphoid organs during and following influenza A virus (IAV) infection. We observed a triphasic production of Ifnγ expression. Unconventional T cells and innate lymphoid cells, particularly NK cells, were the dominant producers of early Ifnγ, while CD4 and CD8 T cells were the main producers by day 10 post-infection. Following viral clearance, some memory CD4 and CD8 T cells continued to express Ifnγ in the lungs and draining lymph node. Interestingly, Ifnγ production by lymph node Natural Killer (NK), NKT and innate lymphoid type 1 cells also continued to be above naïve levels, suggesting memory-like phenotypes for these cells. Analysis of the localisation of Ifnγ+ memory CD4 and CD8 T cells demonstrated that cytokine+ T cells were located near airways and in the lung parenchyma. Following a second IAV challenge, lung IAV specific CD8 T cells rapidly increased their expression of Ifnγ while CD4 T cells in the draining lymph node increased their Ifnγ response. Together, these data suggest that Ifnγ production fluctuates based on cellular source and location, both of which could impact subsequent immune responses

Tolerance induction in memory CD4 T cells is partial and reversible

Memory T cells respond rapidly in part because they are less reliant on heightened levels of costimulatory molecules. This enables rapid control of secondary infecting pathogens but presents challenges to efforts to control or silence memory CD4 T cells, for example in antigen specific tolerance strategies for autoimmunity. We have examined the transcriptional and functional consequences of re‐activating memory CD4 T cells in the absence of an adjuvant. We find that memory CD4 T cells generated by infection or immunisation survive secondary activation with antigen delivered without adjuvant, regardless of their location in secondary lymphoid organs or peripheral tissues. These cells were, however, functionally altered following a tertiary immunisation with antigen and adjuvant, proliferating poorly but maintaining their ability to produce inflammatory cytokines. Transcriptional and cell cycle analysis of these memory CD4 T cells suggest they are unable to commit fully to cell division potentially because of low expression of DNA repair enzymes. In contrast, these memory CD4 T cells could proliferate following tertiary reactivation by viral re‐infection. These data indicate that antigen specific tolerogenic strategies must examine multiple parameters of T cell function, and provide insight into the molecular mechanisms that may lead to deletional tolerance of memory CD4 T cells

T cell response in the lung following influenza virus infection

Methods that enable the identification of virus-specific CD4 and CD8 T cells are key to our understanding of how the adaptive immune response controls viral infection. Here we describe two distinct methods to evaluate the T cell response to influenza A virus (IAV). The number and phenotype of T cells that respond to natural IAV epitopes can be assessed by flow cytometry using MHC class I and class II tetramers. Using this system, IAV-specific T cells can be tracked in various organs within the same animal, or, in different cohorts, the response can be evaluated at several time points following infection. While providing clear quantitative data, flow cytometry cannot provide any information about T cell location within the lung or interactions between responding T cells and other cell types. Here we also describe a method to examine activated CD4 T cells in the lungs of living animals using multiphoton intravital microscopy, thus providing real-time analysis of T cell behavior during an infection