You will be presented with the abstract, introduction, and discussion section from a biomedical research article. Your task is to create a summary that a layperson can understand, capturing the essence of the research article. Keep essential scientific terms, but ensure that the language remains clear and the concepts are explained in an uncomplicated manner.

Title: Central memory CD8+ T cells become CD69+ tissue-residents during viral skin infection independent of CD62L-mediated lymph node surveillance}
Article:
Abstract:
Memory CD8+ T cells in the circulation rapidly infiltrate non-lymphoid tissues following infection and provide protective immunity in an antigen-specific manner. However, the subsequent fate of memory CD8+ T cells after entering non-lymphoid tissues such as the skin during a secondary infection is largely unknown. Furthermore, because expression of CD62L is often used to identify the central memory( TCM) CD8+ T cell subset, uncoupling the physical requirement for CD62L-mediated lymph node homing versus other functional attributes of TCM CD8+ T cells remains unresolved. Here, we show that in contrast to naïve CD8+ T cells, memory CD8+ T cells traffic into the skin independent of CD62L-mediated lymph node re-activation and provide robust protective immunity against Vaccinia virus( VacV) infection. TCM, but not effector memory( TEM), CD8+ T cells differentiated into functional CD69+/CD103- tissue residents following viral clearance, which was also dependent on local recognition of antigen in the skin microenvironment. Finally, we found that memory CD8+ T cells expressed granzyme B after trafficking into the skin and utilized cytolysis to provide protective immunity against VacV infection. Collectively, these findings demonstrate that TCM CD8+ T cells become cytolytic following rapid infiltration of the skin to protect against viral infection and subsequently differentiate into functional CD69+ tissue-residents.
Introduction:
Following acute infection or successful vaccination, antigen-specific CD8+ T cells that survive contraction differentiate into long-lived memory cells that provide protective immunity against re-infection. Memory CD8+ T cells exhibit a variety of functional characteristics( e. g. production of cytokines, cytolytic activity) that allow them to provide protective immunity against viruses and other intracellular pathogens[1, 2]. Another key feature of memory CD8+ T cells is their acquired ability to infiltrate and/or be retained in non-lymphoid tissues, whereas naïve CD8+ T cells are largely restricted to the circulation and secondary lymphoid organs[3, 4]. Memory CD8+ T cells can be broadly classified as those that are present in the circulation and those that remain in non-lymphoid tissues, now referred to as “tissue-resident” memory cells( TRM)[5]. Because of their location, often at environmental barriers, it is believed that pro-inflammatory TRM CD8+ T cells are the cells most responsible for immediate protection against re-infections[6–10], prior to the trafficking of additional circulating memory CD8+ T cells into the site of infection to further promote pathogen clearance[11]. Understanding the mechanisms that control both the formation and protective features of these diverse memory T cell populations remains a challenge for rational vaccine design and development. Both TRM and memory CD8+ T cells in the circulation provide protective immunity against re-infections of the skin and other non-lymphoid tissues. Following T cell receptor( TCR) engagement, TRM CD8+ T cells rapidly produce IFNγ that initiates a local inflammatory response and also promotes a general “anti-viral” state in the tissue microenvironment[12, 13]. Alternatively, in response to inflammatory cues, circulating memory CD8+ T cells rapidly enter non-lymphoid tissues and also provide significant protective immunity against viral skin infection[14, 15]. Memory CD8+ T cells in the circulation are often classified as being either central( TCM) or effector( TEM) memory based on the expression of receptors required for lymph node homing( i. e. CD62L and CCR7)[16]. We have recently demonstrated that TCM CD8+ T cells are the primary memory cell subset that rapidly infiltrates the skin following VacV infection, whereas TEM CD8+ T cells are mostly confined to the circulation[15]. Because CD62L is often used to identify the TCM subset in mice, uncoupling the physical requirement for CD62L-mediated lymph node homing versus other functional attributes of TCM CD8+ T cells during a secondary challenge has, for the most part, been correlative. Furthermore, the protective mechanisms used by circulating memory CD8+ T cells to control viral infection after trafficking into the skin are largely undefined. The role that the tissue microenvironment plays in regulating the differentiation of TRM CD8+ T cells remains controversial. In contrast to systemic viral infections such as lymphocytic choriomeningitis virus( LCMV), infection of the skin with herpes simplex virus-1( HSV-1) or Vaccinia virus( VacV) activates naïve antigen-specific CD8+ T cells that subsequently traffic into and differentiate into TRM almost exclusively at the site of infection[17–19], suggesting that local inflammation, at least in part, is critical for promoting the acquisition of the TRM phenotype. Tissue-resident memory CD8+ T cells are often identified by expression or co-expression of CD69 and CD103( αE integrin) and both of these proteins contribute to the formation of TRM in the skin[20, 21]. When paired with β7 integrin, CD103 becomes a receptor for E-cadherin and it has been proposed that this interaction is critical for retaining CD8+ T cells in the epidermis. An analysis of skin from healthy human subjects agrees with this model, as CD103-expressing CD8+ T cells are enriched in the epidermis compared to the cells that only express CD69[22]. TGFβ-receptor signaling causes effector CD8+ T cells to express CD103, suggesting that the capacity to respond to TGF-β may be an important retention signal that facilitates TRM differentiation in the skin and other E-cadherin-rich non-lymphoid tissues[20, 23]. In contrast, overexpression of genes that promote tissue egress including the transcription factor KLF2 or the sphingosine-1-phosphate receptor( S1P1; S1PR1) antagonizes TRM development[24]. Collectively, these findings suggest that within tissue microenvironments, the ultimate fate of activated CD8+ T cells is highly dependent on the integration of conflicting signals that either promote or inhibit non-lymphoid tissue recruitment and retention. However, the signaling mechanisms within the tissue microenvironment and the cellular origins of either the CD69+ or CD69+/CD103+ TRM subsets in the skin remain largely undefined. In this study, we analyzed the functionality and lineage relationship of memory CD8+ T cell populations following a secondary viral skin infection. We found that circulating memory CD8+ T cells rapidly traffic into the skin following VacV infection independent of their ability to survey peripheral lymph nodes and provide significant protective immunity. In contrast to naïve CD8+ T cells, most circulating TCM CD8+ T cells do not express CD103 in the skin microenvironment during the resolution of viral infection and only a subset ultimately became functional CD69+/CD103- TRM. Finally, after entering VacV-infected skin, circulating memory CD8+ T cells expressed granzyme B and required cytolytic function to provide protection against VacV infection. Altogether, this study demonstrates that TCM CD8+ T cells will rapidly infiltrate non-lymphoid tissues in response to local inflammatory cues, accelerate viral clearance, and become secondary CD69+ TRM that form following the resolution of viral skin infection, an important consideration for tissue-specific vaccine development.
Discussion:
TRM CD8+ T cells are believed to play an important role in host defense against pathogens, but have also been suggested to be the dysfunctional culprit in a variety of inflammatory diseases of the skin and perhaps other non-lymphoid tissues[36, 37]. Thus, an understanding of both the cell intrinsic mechanisms, as well as the cell extrinsic microenvironment inflammatory factors that ultimately control the formation of these cell populations is highly relevant for a variety of diseases and pathological conditions. Most studies to date have focused on the capacity for TRM CD8+ T cells to develop from naïve CD8+ T cells following a primary infection in mice, where no protective adaptive immunity against the pathogen is present. Notably, there are essentially no TRM CD8+ T cells in laboratory mice prior to any type of experimental manipulation, whereas human skin is rich in a heterogeneous mixture of CD8+ and CD4+ T cells exhibiting a variety of phenotypes[22]. This observation suggests that exposure to environmental antigens and pathogens throughout life is continually shaping the composition of the skin T cell repertoire[38]. Because most of our current knowledge of the mechanisms controlling TRM formation have been studied in the context of a primary infection, understanding how pre-existing immunological memory and dynamic host-pathogen interactions within the local tissue microenvironment impact TRM differentiation has remained largely unknown. Recent studies by us and others suggest that antigen recognition in non-lymphoid tissues such as the skin and lung will significantly increase the formation of TRM following a primary infection[18, 39–42]. Indeed, we also found that memory CD8+ T cells became enriched in the skin microenvironment where antigen recognition occurred during a secondary challenge. Interestingly, however, we also found that most circulating memory CD8+ T cells that infiltrate the skin during VacV infection express CD69 following antigen recognition, but are unable to express CD103 when stimulated with TGF-β. Although the generation of a local inflammatory environment is clearly essential for T cell recruitment prior to TRM differentiation, it has also been demonstrated that antigen recognition and some pro-inflammatory cytokines such as IL-12 and IFNβ will inhibit CD103 expression[23, 43]. In contrast to our findings, it has recently been reported that CD69+/CD103+ TRM CD8+ T cells are generated from circulating memory CD8+ T cells following HSV-1 skin infection[44]. Interestingly, we and others have shown that local antigen recognition in the skin is critical for TRM CD8+ T cells to form following poxvirus infection[18, 39, 45], whereas antigen-specific TRM formation does not seem to occur during skin infection with HSV-1[9]. Because here we also show that CD69+ TRM are generated by circulating memory T cells in an antigen-specific manner, this suggests that antigen recognition in the skin is critical for the formation of this population following poxvirus infection, but may not form and/or be retained during HSV-1 skin infection. Thus, it seems that both antigen recognition and the cytokine milieu within the tissue microenvironment act in concert to ultimately control the extent of TRM differentiation, but how these diverse signaling pathways are integrated in vivo is still largely undefined. Nevertheless, our data support a model where expression of CD69 identifies skin TRM that received a local antigen recognition event, but not a TGF-β( or perhaps additional cytokines) signal to transition into a CD103+/CD69+ TRM. Our study also highlights that the capacity for memory CD8+ T cells to home into and survey peripheral lymph nodes does not play a major role during a secondary recall response against a local skin infection and that the primary feature of TCM that allows this memory subset to provide protective immunity is their capacity to rapidly and specifically traffic into inflamed non-lymphoid tissues. Another recent report demonstrated that KLRG1+, CX3CR1+ TEM CD8+ T cells are essentially confined to the circulation and are unable to infiltrate non-lymphoid tissues during steady-state homeostatic trafficking[46]. In addition, it has also been recently reported that TCM CD8+ and CD4+ T cells from humans express significant levels of non-lymphoid tissue-homing addressins and induce skin inflammation in a mouse model of dermatitis[47]. These studies agree with our data demonstrating that TCM are the major tissue trafficking subset that provides protection against VacV skin infection[15] and also are precursors to CD69+ TRM. In contrast, highly cytolytic TEM CD8+ T cells provide more protective immunity than TCM against systemic infections that reach the circulation and/or spleen( i. e., when pathogens are delivered to mice intravenously or into the peritoneal cavity)[48, 49]. Interestingly, we found that circulating memory CD8+ T cells also require cytolysis to control VacV infection of the skin and limit inflammation, which is in contrast to TRM CD8+ T cells, which have been proposed to provide their protective benefit through local production of IFNγ[12, 13]. It will be of interest to determine whether circulating memory CD8+ T cells use cytolysis as the primary mechanism to control other intracellular pathogens in non-lymphoid tissues or if the production of cytokines( e. g. IFNγ, TNFα) is required to protect against some types of infections. Altogether, our findings suggest that the circulating TCM CD8+ T cells function as the first responders against infections in peripheral tissues, ultimately controlling and eliminating viral infection. In summary, our results demonstrate that TCM CD8+ T cells traffic into non-lymphoid tissue microenvironments following viral infection in a CD62L-independent fashion and protect against poxvirus using perforin-mediated cytotoxicity. Our finding that the majority of the memory CD8+ T cells initially recruited into the site of viral infection( by day 3) had not proliferated strongly suggests that the primary feature of TCM that allows this memory subset to provide rapid protective immunity is their ability to infiltrate inflamed non-lymphoid tissues, rather than the re-activation/proliferative expansion in draining lymph nodes. In fact, recent experimental evidence suggests that terminally differentiated TEM CD8+ T cells are largely confined to the circulation and not actively surveying non-lymphoid tissues[15, 46, 50] and thus, the long-standing model describing the trafficking potentials of TCM and TEM CD8+ T cells subsets continues to be refined. Overall, our findings provide new insight regarding the protective mechanisms used by circulating memory CD8+ T cells during a secondary viral infection in peripheral tissues, but also reveals important information about how “prime-boost” vaccination strategies of generating circulating memory CD8+ T cells followed by a local heterologous challenge will generate antigen-specific secondary memory T cell populations in non-lymphoid tissues such as the skin.
Following vaccination or resolution of viral infection , three distinct populations of virus-specific memory CD8+ T cells are generated . Central memory cells ( TCM ) circulate between blood and lymph nodes and are able to rapidly enter non-lymphoid tissues in response to infection or tissue injury . In contrast , effector memory ( TEM ) are largely restricted to the blood and tissue-resident memory cells ( TRM ) are primarily confined within non-lymphoid tissues . In this study , we set out to interrogate the functionality and lineage relationship of these memory populations following a secondary viral skin infection . Circulating memory CD8+ T cells rapidly infiltrated the skin following viral infection and used cytolysis to provide robust protective immunity . TCM , but not TEM , trafficked into the skin and differentiated into functional TRM following resolution of viral infection and this occurred independent of the ability of these cells to home into and survey peripheral lymph nodes . Altogether , our findings demonstrate that a defined lineage of circulating memory CD8+ T cells are the principle responders against viral skin infection and also the precursors to secondary TRM CD8+ T cells that then form , which are important considerations for understanding host defense mechanisms against recurring viral infections and tissue-specific vaccine development .