Recall Responses from Brain-Resident Memory CD8+ T Cells (bTRM) Induce Reactive Gliosis

摘要

class="head no_bottom_margin" id="sec1title">IntroductionIt is currently unknown why HIV-associated neurocognitive disorders (HAND) persist despite effective viral suppression to undetectable levels in most combination antiretroviral therapy (cART)-treated individuals. Although patients on successful cART show sustained viral suppression, as indicated by routine plasma monitoring and occasional cerebrospinal fluid (CSF) monitoring, “blips” indicative of transient HIV replication are often detected with more frequent testing, as reviewed in ). A number of studies link persistent immune activation, neuroinflammation, and CSF viral escape in cART-treated individuals to increased disease progression, as well as an increased risk for HAND (, , ). Interruption of cART has been associated with CSF viral escape and neuronal injury, as well as with worsened neurocognitive performance (, , ). Although fulminant encephalitis is now uncommon in HIV-infected patients, biomarkers of continuing neuroinflammation in the CSF and brain, as well as neuronal injury, are frequently detected in virally suppressed individuals. Still, partial protection against HAND by successful cART does indicate some direct link to HIV replication. Persistent HIV infection and reactivation from central nervous system (CNS) reservoirs, even if intermittent, appears likely in cART-experienced patients (). Brain infection may also be re-seeded through blood or through recently described meningeal lymphatics (). In addition, evidence for the involvement of CD8⁺ T cells, especially through production of interferon (IFN)-γ, in CNS pathogenesis in patients who received cART continues to mount (). Therefore, CSF viral escape and the associated production of viral antigen (Ag), as well as its generation of subsequent adaptive, recall responses by brain-resident CD8⁺ T cells to control viral spread, may induce neuroinflammation that drives bystander CNS injury.Resolution of adaptive immune responses and generation of immunological memory is an essential process to confer long-term protective immunity, particularly in tissues like the brain. The presence of CD8⁺ T cells in post-mortem brains of HIV-infected patients, as well as in the brains of animal models, is well documented (, , , , , href="#bib35" rid="bib35" class=" bibr popnode">Marcondes et al., 2007, href="#bib53" rid="bib53" class=" bibr popnode">Schrier et al., 2015). Recent studies have demonstrated that following clearance of many acute viral infections, CD8⁺ T lymphocytes generate a population of long-lived, non-recirculating tissue-resident memory T cells (TRM) in non-lymphoid tissue, and it is becoming increasingly clear that these TRM play critical roles in controlling re-encountered pathogens and accelerating the process of clearance (href="#bib32" rid="bib32" class=" bibr popnode">Mackay et al., 2013, href="#bib38" rid="bib38" class=" bibr popnode">Masopust et al., 2010, href="#bib46" rid="bib46" class=" bibr popnode">Park and Kupper, 2015, href="#bib52" rid="bib52" class=" bibr popnode">Schenkel and Masopust, 2014). It is well established that during acute viral infection, most pathogens are rapidly cleared by the generation of a large number of short-lived effector T cells (SLEC), which die via apoptosis once cognate Ag is cleared. Simultaneously, the T cell response is also triggered to generate a defined subset identified as memory precursor effector cells (MPEC). These MPEC begin to develop into a TRM phenotype shortly after infection. Recent work by several groups provides evidence that there is a clear distinction between terminal effector and memory cells based on heterogeneity in expression of killer cell lectin-like receptor G1 (KLRG1) (href="#bib2" rid="bib2" class=" bibr popnode">Bengsch et al., 2007, href="#bib23" rid="bib23" class=" bibr popnode">Kaech and Wherry, 2007, href="#bib69" rid="bib69" class=" bibr popnode">Yuzefpolskiy et al., 2015).We have recently characterized brain-infiltrating T cells that persist within the CNS after acute murine cytomegalovirus (MCMV) infection (href="#bib48" rid="bib48" class=" bibr popnode">Prasad et al., 2015). We have also shown that these brain CD8⁺ T cell populations shift from SLEC that clear infection to MPEC that protect against viral reactivation and re-challenge. The shift of prominent SLEC populations to MPEC populations is concomitant with transition from acute through chronic phases of infection. TRM are characterized by their non-recirculating, resident nature in tissues. It has been well reported that TRM often express αEβ7 integrin, and integrin αE, otherwise known as CD103, is used as a marker of particular types of TRM. High expression of CD103 and CD69 is a common feature of resident memory cells observed in epithelial tissue, as well as a phenotypic signature of bTRM (href="#bib59" rid="bib59" class=" bibr popnode">Steinbach et al., 2016, href="#bib64" rid="bib64" class=" bibr popnode">Wakim et al., 2010, href="#bib65" rid="bib65" class=" bibr popnode">Watanabe et al., 2015, href="#bib67" rid="bib67" class=" bibr popnode">Woon et al., 2016). In contrast, effector and memory cells in circulation appear to lack expression of both CD103 and CD69 (href="#bib17" rid="bib17" class=" bibr popnode">Gebhardt and Mackay, 2012, href="#bib37" rid="bib37" class=" bibr popnode">Masopust et al., 2006). It has also been shown that CD69 expression is required for optimal formation of TRM following herpes simplex virus infection in tissues such as the skin and dorsal root ganglia (href="#bib32" rid="bib32" class=" bibr popnode">Mackay et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Mackay et al., 2015b). In addition to CD103⁺ and CD69⁺ markers, TRM are often reported to express CD49a. CD49a constitutes the α subunit of α1β1 integrin receptor, also known as very late Ag 1 (VLA-1) (href="#bib13" rid="bib13" class=" bibr popnode">Cheuk et al., 2017). Experiments using skin, lung, and gut show differential expression of CCR7, as well as CXCR3, which define migration properties of T cells (href="#bib43" rid="bib43" class=" bibr popnode">Mueller and Mackay, 2016, href="#bib55" rid="bib55" class=" bibr popnode">Slutter et al., 2013). Although imperfect, expression of these markers is frequently used to identify TRM populations when stringent migration studies, such as parabiosis, are not feasible, reviewed in href="#bib52" rid="bib52" class=" bibr popnode">Schenkel and Masopust (2014). Finally, protection of the CNS from reinfection by lymphocytic choriomeningitis virus (LCMV) was found to depend on these brain-resident memory CD8⁺ T cells (bTRM) (href="#bib59" rid="bib59" class=" bibr popnode">Steinbach et al., 2016).Given their significance in antiviral defense, surprisingly little is known about this brain-resident memory cell population in the context of HAND. Ag-specific lymphocytes residing within tissues are uniquely poised to respond rapidly to their cognate Ag, and TRM functions extend far beyond cytotoxic T lymphocyte activity. In several infection models, it is clear that recall responses of TRM to small amounts of Ag result in production of IFN-γ (href="#bib40" rid="bib40" class=" bibr popnode">McMaster et al., 2015, href="#bib59" rid="bib59" class=" bibr popnode">Steinbach et al., 2016). Furthermore, this IFN-γ production results in IFN-stimulated gene expression in surrounding cells, thereby amplifying the activation of a small number of adaptive immune cells into an organ-wide antiviral response (href="#bib1" rid="bib1" class=" bibr popnode">Ariotti et al., 2014). Similarly, activation of adaptive immune responses from TRM has been shown to stimulate protective, innate responses in the LCMV model (href="#bib52" rid="bib52" class=" bibr popnode">Schenkel and Masopust, 2014). From these studies, it is clear that a small number of TRM accelerate pathogen control in the event of reinfection or reactivation of latent infection by instructing tissue-resident innate immune cells, such as microglia. Yet there have been no definitive experiments to evaluate neurotoxic consequences of anti-HIV recall immune response by bTRM in driving tissue-wide activation of brain-resident microglial cells, its associated neurotoxicity, and development of subsequent neurocognitive impairment. Here, we demonstrate that HIV-specific T_RM are elicited within brain using a heterologous prime-CNS boost strategy. We show that Ag-specific CD8⁺ bT_RM are established, which recognize an immunodominant viral epitope. Subsequent recall responses to specific epitope peptides resulted in striking upregulation of major histocompatibility complex (MHC) class II (MHC-II) and programmed death-ligand 1 (PD-L1) on microglial cells, indicative of neuroinflammation. Taken together, data presented here show that recall responses from bT_RM can induce reactive gliosis.