Temporal Changes in Innate and Adaptive Immunity During Sepsis as Determined by ELISpot.

BACKGROUND: The inability to evaluate host immunity in a rapid quantitative manner in patients with sepsis has severely hampered development of novel immune therapies. The ELISpot assay is a functional bioassay that measures the number of cytokine-secreting cells and the relative amount of cytokine produced at the single-cell level. A key advantage of ELISpot is its excellent dynamic range enabling a more precise quantifiable assessment of host immunity. Herein, we tested the hypothesis that the ELISpot assay can detect dynamic changes in both innate and adaptive immunity as they often occur during sepsis. We also tested whether ELISpot could detect the effect of immune drug therapies to modulate innate and adaptive immunity. METHODS: Mice were made septic using sublethal cecal ligation and puncture (CLP). Blood and spleens were harvested serially and ex vivo IFN-γ and TNF-α production were compared by ELISpot and ELISA. The capability of ELISpot to detect changes in innate and adaptive immunity due to in vivo immune therapy with dexamethasone, IL-7, and arginine was also evaluated. RESULTS: ELISpot confirmed a decreased innate and adaptive immunity responsiveness during sepsis progression. More importantly, ELISpot was also able to detect changes in adaptive and innate immunity in response to immune-modulatory reagents, for example dexamethasone, arginine, and IL-7 in a readily quantifiable manner, as predicted by the reagents known mechanisms of action. ELISpot and ELISA results tended to parallel one another although some differences were noted. CONCLUSION: ELISpot offers a unique capability to assess the functional status of both adaptive and innate immunity over time. The results presented herein demonstrate that ELISpot can also be used to detect and follow the in vivo effects of drugs to ameliorate sepsis-induced immune dysfunction. This capability would be a major advance in guiding new immune therapies in sepsis.

Age-associated accumulation of B cells promotes macrophage inflammation and inhibits lipolysis in adipose tissue during sepsis.

Non-canonical lipolysis induced by inflammatory cytokines or Toll-like receptor ligands is required for the regulation of inflammation during endotoxemia and sepsis. Canonical lipolysis induced by catecholamines declines during aging due to factors including an expansion of lymphocytes, pro-inflammatory macrophage polarization, and an increase in chronic low-grade inflammation; however, the extent to which the non-canonical pathway of lipolysis is active and impacted by immune cells during aging remains unclear. Therefore, we aimed to define the extent to which immune cells from old mice influence non-canonical lipolysis during sepsis. We identified age-associated impairments of non-canonical lipolysis and an accumulation of dysfunctional B1 B cells in the visceral white adipose tissue (vWAT) of old mice. Lifelong deficiency of B cells results in restored non-canonical lipolysis and reductions in pro-inflammatory macrophage populations. Our study suggests that targeting the B cell-macrophage signaling axis may resolve metabolic dysfunction in aged vWAT and attenuate septic severity in older individuals.

Mouse Models of Sepsis.

Human sepsis is a complex disease that manifests with a diverse range of phenotypes and inherent variability among individuals, making it hard to develop a comprehensive animal model. Despite this difficulty, numerous models have been developed that capture many key aspects of human sepsis. The robustness of these models is vital for conducting pre-clinical studies to test and develop potential therapeutics. In this article, we describe four different models of murine sepsis that can be used to address different scientific questions relevant to the pathology and immune response during and after a septic event. Basic Protocol 1 details a non-synchronous cecal ligation and puncture (CLP) model of sepsis, where mice are subjected to polymicrobial exposure through surgery at different time points within 2 weeks. This variation in sepsis onset establishes each mouse at a different state of inflammation and cytokine levels that mimics the variability observed in humans when they present in the clinic. This model is ideal for studying the long-term impact of sepsis on the host. Basic Protocol 2 is also a type of polymicrobial sepsis, where injection of a specific amount of cecal slurry from a donor mouse into the peritoneum of recipient mice establishes immediate inflammation and sepsis without any need for surgery. Basic Protocol 3 describes infecting mice with a defined gram-positive or -negative bacterial strain to model a subset of sepsis observed in humans infected with a single pathogen. Basic Protocol 4 describes administering LPS to induce sterile endotoxemia. This form of sepsis is observed in humans exposed to bacterial toxins from the environment. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Non-synchronous cecal ligation and puncture Basic Protocol 2: Cecal slurry model of murine sepsis Basic Protocol 3: Monomicrobial model of murine sepsis Basic Protocol 4: LPS model of murine sepsis.

Regenerating murine CD8+ lung tissue resident memory T cells after targeted radiation exposure.

Radiation exposure occurs during medical procedures, nuclear accidents, or spaceflight, making effective medical countermeasures a public health priority. Naïve T cells are highly sensitive to radiation-induced depletion, although their numbers recover with time. Circulating memory CD8+ T cells are also depleted by radiation; however, their numbers do not recover. Critically, the impact of radiation exposure on tissue-resident memory T cells (TRM) remains unknown. Here, we found that sublethal thorax-targeted radiation resulted in the rapid and prolonged numerical decline of influenza A virus (IAV)-specific lung TRM in mice, but no decline in antigen-matched circulating memory T cells. Prolonged loss of lung TRM was associated with decreased heterosubtypic immunity. Importantly, boosting with IAV-epitope expressing pathogens that replicate in the lungs or peripheral tissues or with a peripherally administered mRNA vaccine regenerated lung TRM that was derived largely from circulating memory CD8+ T cells. Designing effective vaccination strategies to regenerate TRM will be important in combating the immunological effects of radiation exposure.

Adverse outcomes and an immunosuppressed endotype in septic patients with reduced IFN-γ ELISpot.

BACKGROUNDSepsis remains a major clinical challenge for which successful treatment requires greater precision in identifying patients at increased risk of adverse outcomes requiring different therapeutic approaches. Predicting clinical outcomes and immunological endotyping of septic patients generally relies on using blood protein or mRNA biomarkers, or static cell phenotyping. Here, we sought to determine whether functional immune responsiveness would yield improved precision.METHODSAn ex vivo whole-blood enzyme-linked immunosorbent spot (ELISpot) assay for cellular production of interferon γ (IFN-γ) was evaluated in 107 septic and 68 nonseptic patients from 5 academic health centers using blood samples collected on days 1, 4, and 7 following ICU admission.RESULTSCompared with 46 healthy participants, unstimulated and stimulated whole-blood IFN-γ expression was either increased or unchanged, respectively, in septic and nonseptic ICU patients. However, in septic patients who did not survive 180 days, stimulated whole-blood IFN-γ expression was significantly reduced on ICU days 1, 4, and 7 (all P < 0.05), due to both significant reductions in total number of IFN-γ-producing cells and amount of IFN-γ produced per cell (all P < 0.05). Importantly, IFN-γ total expression on days 1 and 4 after admission could discriminate 180-day mortality better than absolute lymphocyte count (ALC), IL-6, and procalcitonin. Septic patients with low IFN-γ expression were older and had lower ALCs and higher soluble PD-L1 and IL-10 concentrations, consistent with an immunosuppressed endotype.CONCLUSIONSA whole-blood IFN-γ ELISpot assay can both identify septic patients at increased risk of late mortality and identify immunosuppressed septic patients.TRIAL REGISTRYN/A.FUNDINGThis prospective, observational, multicenter clinical study was directly supported by National Institute of General Medical Sciences grant R01 GM-139046, including a supplement (R01 GM-139046-03S1) from 2022 to 2024.

Defining Parameters That Modulate Susceptibility and Protection to Respiratory Murine Coronavirus MHV1 Infection.

Patients infected with SARS-CoV-2 experience variable disease susceptibility, and patients with comorbidities such as sepsis are often hospitalized for COVID-19 complications. However, the extent to which initial infectious inoculum dose determines disease outcomes and whether this can be used for immunological priming in a genetically susceptible host has not been completely defined. We used an established SARS-like murine model in which responses to primary and/or secondary challenges with murine hepatitis virus type 1 (MHV-1) were analyzed. We compared the response to infection in genetically susceptible C3H/HeJ mice, genetically resistant C57BL/6J mice, and genetically diverse, variably susceptible outbred Swiss Webster mice. Although defined as genetically susceptible to MHV-1, C3H/HeJ mice displayed decreasing dose-dependent pathological changes in disease severity and lung infiltrate/edema, as well as lymphopenia. Importantly, an asymptomatic dose (500 PFU) was identified that yielded no measurable morbidity/mortality postinfection in C3H/HeJ mice. Polymicrobial sepsis induced via cecal ligation and puncture converted asymptomatic infections in C3H/HeJ and C57BL/6J mice to more pronounced disease, modeling the impact of sepsis as a comorbidity to ß-coronavirus infection. We then used low-dose infection as an immunological priming event in C3H/HeJ mice, which provided neutralizing Ab-dependent, but not circulating CD4/CD8 T cell-dependent, protection against a high-dose MHV-1 early rechallenge. Together, these data define how infection dose, immunological status, and comorbidities modulate outcomes of primary and secondary ß-coronavirus infections in hosts with variable susceptibility.

CD115+ monocytes protect microbially experienced mice against E. coli-induced sepsis.

Uropathogenic E. coli (UPEC) is a primary organism responsible for urinary tract infections and a common cause of sepsis. Microbially experienced laboratory mice, generated by cohousing with pet store mice, exhibit increased morbidity and mortality to polymicrobial sepsis or lipopolysaccharide challenge. By contrast, cohoused mice display significant resistance, compared with specific pathogen-free mice, to a monomicrobial sepsis model using UPEC. CD115+ monocytes mediate protection in the cohoused mice, as depletion of these cells leads to increased mortality and UPEC pathogen burden. Further study of the cohoused mice reveals increased TNF-α production by monocytes, a skewing toward Ly6ChiCD115+ "classical" monocytes, and enhanced egress of Ly6ChiCD115+ monocytes from the bone marrow. Analysis of cohoused bone marrow also finds increased frequency and number of myeloid multipotent progenitor cells. These results show that a history of microbial exposure impacts innate immunity in mice, which can have important implications for the preclinical study of sepsis.

Sepsis leads to lasting changes in phenotype and function of naïve CD8 T cells.

Sepsis, an amplified immune response to systemic infection, is characterized by a transient cytokine storm followed by chronic immune dysfunction. Consequently, sepsis survivors are highly susceptible to newly introduced infections, suggesting sepsis can influence the function and composition of the naïve CD8 T cell pool and resulting pathogen-induced primary CD8 T cell responses. Here, we explored the extent to which sepsis induces phenotypic and functional changes within the naïve CD8 T cell pool. To interrogate this, the cecal ligation and puncture (CLP) mouse model of polymicrobial sepsis was used. In normal, non-septic mice, we show type-I interferon (IFN I)-mediated signaling plays an important role in driving the phenotypic and functional heterogeneity in the naïve CD8 T cell compartment leading to increased representation of Ly6C+ naïve CD8 T cells. In response to viral infection after sepsis resolution, naïve Ly6C+ CD8 T cells generated more primary effector and memory CD8 T cells with slower conversion to a central memory CD8 T cell phenotype (Tcm) than Ly6C- naïve CD8 T cells. Importantly, as a potent inducer of cytokine storm and IFN I production, sepsis leads to increased representation of Ly6C+ naïve CD8 T cells that maintained their heightened ability to respond (i.e., effector and memory CD8 T cell accumulation and cytokine production) to primary LCMV infection. Lastly, longitudinal analyses of peripheral blood samples obtained from septic patients revealed profound changes in CD8 T cell subset composition and frequency compared to healthy controls. Thus, sepsis has the capacity to alter the composition of naïve CD8 T cells, directly influencing primary CD8 T cell responses to newly introduced infections.

Adverse Long-Term Outcomes and an Immune Suppressed Endotype in Sepsis Patients with Reduced Interferon-γELISpot: A Multicenter, Prospective Observational Study.

BACKGROUND: Sepsis remains a major clinical challenge for which successful treatment requires greater precision in identifying patients at increased risk of adverse outcomes requiring different therapeutic approaches. Predicting clinical outcomes and immunological endotyping of septic patients has generally relied on using blood protein or mRNA biomarkers, or static cell phenotyping. Here, we sought to determine whether functional immune responsiveness would yield improved precision. METHODS: An ex vivo whole blood enzyme-linked immunosorbent (ELISpot) assay for cellular production of interferon-γ (IFN-γ) was evaluated in 107 septic and 68 non-septic patients from five academic health centers using blood samples collected on days 1, 4 and 7 following ICU admission. RESULTS: Compared with 46 healthy subjects, unstimulated and stimulated whole blood IFNγ expression were either increased or unchanged, respectively, in septic and nonseptic ICU patients. However, in septic patients who did not survive 180 days, stimulated whole blood IFNγ expression was significantly reduced on ICU days 1, 4 and 7 (all p<0.05), due to both significant reductions in total number of IFNγ producing cells and amount of IFNγ produced per cell (all p<0.05). Importantly, IFNγ total expression on day 1 and 4 after admission could discriminate 180-day mortality better than absolute lymphocyte count (ALC), IL-6 and procalcitonin. Septic patients with low IFNγ expression were older and had lower ALC and higher sPD-L1 and IL-10 concentrations, consistent with an immune suppressed endotype. CONCLUSIONS: A whole blood IFNγ ELISpot assay can both identify septic patients at increased risk of late mortality, and identify immune-suppressed, sepsis patients.

The Calm after the Storm: Implications of Sepsis Immunoparalysis on Host Immunity.

The immunological hallmarks of sepsis include the inflammation-mediated cytokine storm, apoptosis-driven lymphopenia, and prolonged immunoparalysis. Although early clinical efforts were focused on increasing the survival of patients through the first phase, studies are now shifting attention to the long-term effects of sepsis on immune fitness in survivors. In particular, the most pertinent task is deciphering how the immune system becomes suppressed, leading to increased incidence of secondary infections. In this review, we introduce the contribution of numerical changes and functional reprogramming within innate (NK cells, dendritic cells) and adaptive (T cells, B cells) immune cells on the chronic immune dysregulation in the septic murine and human host. We briefly discuss how prior immunological experience in murine models impacts sepsis severity, immune dysfunction, and clinical relevance. Finally, we dive into how comorbidities, specifically autoimmunity and cancer, can influence host susceptibility to sepsis and the associated immune dysfunction.

Sublethal whole-body irradiation induces permanent loss and dysfunction in pathogen-specific circulating memory CD8 T cell populations.

The increasing use of nuclear energy sources inevitably raises the risk of accidental or deliberate radiation exposure and associated immune dysfunction. However, the extent to which radiation exposure impacts memory CD8 T cells, potent mediators of immunity to recurring intracellular infections and malignancies, remains understudied. Using P14 CD8 T cell chimeric mice (P14 chimeras) with an lymphocytic choriomeningitis virus (LCMV) infection model, we observed that sublethal (5Gy) whole-body irradiation (WBI) induced a rapid decline in the number of naive (TN) and P14 circulating memory CD8 T cells (TCIRCM), with the former being more susceptible to radiation-induced numeric loss. While TN cell numbers rapidly recovered, as previously described, the number of P14 TCIRCM cells remained low at least 9 mo after radiation exposure. Additionally, the remaining P14 TCIRCM in irradiated hosts exhibited an inefficient transition to a central memory (CD62L+) phenotype compared to nonirradiated P14 chimeras. WBI also resulted in long-lasting T cell intrinsic deficits in memory CD8 T cells, including diminished cytokine and chemokine production along with impaired secondary expansion upon cognate Ag reencounter. Irradiated P14 chimeras displayed significantly higher bacterial burden after challenge with Listeria monocytogenes expressing the LCMV GP33-41 epitope relative to nonirradiated controls, likely due to radiation-induced numerical and functional impairments. Taken together, our findings suggest that sublethal radiation exposure caused a long-term numerical, impaired differentiation, and functional dysregulation in preexisting TCIRCM, rendering previously protected hosts susceptible to reinfection.

Cutting Edge: Influenza-Induced CD11alo Airway CD103+ Tissue Resident Memory T Cells Exhibit Compromised IFN-γ Production after In Vivo TCR Stimulation.

Although tissue resident memory T cells (TRM) in the lung confer robust protection against secondary influenza infection, their in vivo production of IFN-γ is unknown. In this study, using a mouse model, we evaluated production of IFN-γ by influenza-induced TRM (defined as CD103+) that localize to the airways or lung parenchyma. Airway TRM consist of both CD11ahi and CD11alo populations, with low CD11a expression signifying prolonged airway residence. In vitro, high-dose peptide stimulation evoked IFN-γ from most CD11ahi airway and parenchymal TRM, whereas most CD11alo airway TRM did not produce IFN-γ. In vivo production of IFN-γ was clearly detectable in CD11ahi airway and parenchymal TRM but essentially absent in CD11alo airway TRM, irrespective of airway-instilled peptide concentration or influenza reinfection. The majority of IFN-γ-producing airway TRM in vivo were CD11ahi, suggesting recent airway entry. These results question the contribution of long-term CD11alo airway TRM to influenza immunity and reinforce the importance of defining TRM tissue compartment-specific contributions to protective immunity.

Sepsis-induced changes in differentiation, maintenance, and function of memory CD8 T cell subsets.

Formation of long-lasting memory lymphocytes is one of the foundational characteristics of adaptive immunity and the basis of many vaccination strategies. Following the rapid expansion and contraction of effector CD8 T cells, the surviving antigen (Ag)-specific cells give rise to the memory CD8 T cells that persist for a long time and are phenotypically and functionally distinct from their naïve counterparts. Significant heterogeneity exists within the memory CD8 T cell pool, as different subsets display distinct tissue localization preferences, cytotoxic ability, and proliferative capacity, but all memory CD8 T cells are equipped to mount an enhanced immune response upon Ag re-encounter. Memory CD8 T cells demonstrate numerical stability under homeostatic conditions, but sepsis causes a significant decline in the number of memory CD8 T cells and diminishes their Ag-dependent and -independent functions. Sepsis also rewires the transcriptional profile of memory CD8 T cells, which profoundly impacts memory CD8 T cell differentiation and, ultimately, the protective capacity of memory CD8 T cells upon subsequent stimulation. This review delves into different aspects of memory CD8 T cell subsets as well as the immediate and long-term impact of sepsis on memory CD8 T cell biology.

Inefficient Recovery of Repeatedly Stimulated Memory CD8 T Cells after Polymicrobial Sepsis Induction Leads to Changes in Memory CD8 T Cell Pool Composition.

Long-lasting sepsis-induced immunoparalysis has been principally studied in primary (1°) memory CD8 T cells; however, the impact of sepsis on memory CD8 T cells with a history of repeated cognate Ag encounters is largely unknown but important in understanding the role of sepsis in shaping the pre-existing memory CD8 T cell compartment. Higher-order memory CD8 T cells are crucial in providing immunity against common pathogens that reinfect the host or are generated by repeated vaccination. In this study, we analyzed peripheral blood from septic patients and show that memory CD8 T cells with defined Ag specificity for recurring CMV infection proliferate less than bulk populations of central memory CD8 T cells. Using TCR-transgenic T cells to generate 1° and higher-order (quaternary [4°]) memory T cells within the same host, we demonstrate that the susceptibility and loss of both memory subsets are similar after sepsis induction, and sepsis diminished Ag-dependent and -independent (bystander) functions of these memory subsets equally. Both the 1° and 4° memory T cell populations proliferated in a sepsis-induced lymphopenic environment; however, due to the intrinsic differences in baseline proliferative capacity, expression of receptors (e.g., CD127/CD122), and responsiveness to homeostatic cytokines, 1° memory T cells become overrepresented over time in sepsis survivors. Finally, IL-7/anti-IL-7 mAb complex treatment early after sepsis induction preferentially rescued the proliferation and accumulation of 1° memory T cells, whereas recovery of 4° memory T cells was less pronounced. Thus, inefficient recovery of repeatedly stimulated memory cells after polymicrobial sepsis induction leads to changes in memory T cell pool composition, a notion with important implications in devising strategies to recover the number and function of pre-existing memory CD8 T cells in sepsis survivors.

Inflammation Controls Susceptibility of Immune-Experienced Mice to Sepsis.

Sepsis, an amplified immune response to systemic infection that leads to life-threatening organ dysfunction, affects >125,000 people/day worldwide with 20% mortality. Modest therapeutic progress for sepsis has been made, in part because of the lack of therapeutic translatability between mouse-based experimental models and humans. One potential reason for this difference stems from the extensive use of immunologically naive specific pathogen-free mice in preclinical research. To address this issue, we used sequential infections with well-defined BSL-2 pathogens to establish a novel immune-experienced mouse model (specific pathogen experienced [SPexp]) to determine the extent to which immunological experience and/or inflammation influences the host capacity to respond to subsequent infections, including sepsis. Consistent with their immunological experience, SPexp inbred or outbred mice had significant changes in the composition and activation status of multiple leukocyte populations known to influence the severity of cecal ligation and puncture-induced sepsis. Importantly, by varying the timing of sepsis induction, we found the level of basal inflammation controls sepsis-induced morbidity and mortality in SPexp mice. In addition, although a beneficial role of NK cells in sepsis was recently demonstrated in specific pathogen-free mice, NK cell depletion before cecal ligation and puncture induction in SPexp mice lead to diminished mortality, suggesting NK cells may have beneficial or detrimental roles in the response to septic insult dependent on host immune status. Thus, data highlight the importance of utilizing immune-experienced models for preclinical studies to interrogate the cellular/molecular mechanism(s) that could be therapeutically exploited during severe and dysregulated infection-induced inflammatory responses, such as sepsis.

Tcf1 preprograms the mobilization of glycolysis in central memory CD8+ T cells during recall responses.

The mechanisms underlying the heightened protection mediated by central memory CD8+ T (TCM) cells remain unclear. Here we show that the transcription factor Tcf1 was required in resting TCM cells to generate secondary effector CD8+ T cells and to clear pathogens during recall responses. Recall stimulation of CD8+ TCM cells caused extensive reprogramming of the transcriptome and chromatin accessibility, leading to rapid induction of glycolytic enzymes, cell cycle regulators and transcriptional regulators, including Id3. This cluster of genes did not require Tcf1 in resting CD8+ TCM cells, but depended on Tcf1 for optimal induction and chromatin opening in recall-stimulated CD8+ TCM cells. Tcf1 bound extensively to these recall-induced gene loci in resting CD8+ TCM cells and mediated chromatin interactions that positioned these genes in architectural proximity with poised enhancers. Thus, Tcf1 preprogramed a transcriptional program that supported the bioenergetic and proliferative needs of CD8+ TCM cells in case of a secondary challenge.

Novel Mouse Model of Murine Cytomegalovirus-Induced Adaptive NK Cells.

NK cells are important mediators of viral control with the capacity to form adaptive immune features following infection. However, studies of infection-induced adaptive NK cells require adoptive cell transfer to lower the precursor frequency of "Ag-specific" NK cells, potentially limiting the diversity of the NK cell response. In seeking an unmanipulated model to probe the adaptive NK cells, we interrogated a wide range of Collaborative Cross (CC) inbred mice, inbred mouse strains that exhibit broad genetic diversity across strains. Our assessment identified and validated a putative "ideal" CC strain, CC006, which does not require manipulation to generate and maintain adaptive NK cells. Critically, CC006 mice, in contrast to C57BL/6 mice, are capable of developing enhanced NK cell-mediated protective responses to murine CMV infection following m157-mediated vaccination. This work both furthers our understanding of adaptive NK cells and demonstrates the utility of CC mice in the development and interrogation of immunologic models.

Expeditious recruitment of circulating memory CD8 T cells to the liver facilitates control of malaria.

Circulating memory CD8 T cell trafficking and protective capacity during liver-stage malaria infection remains undefined. We find that effector memory CD8 T cells (Tem) infiltrate the liver within 6 hours after malarial or bacterial infections and mediate pathogen clearance. Tem recruitment coincides with rapid transcriptional upregulation of inflammatory genes in Plasmodium-infected livers. Recruitment requires CD8 T cell-intrinsic LFA-1 expression and the presence of liver phagocytes. Rapid Tem liver infiltration is distinct from recruitment to other non-lymphoid tissues in that it occurs both in the absence of liver tissue resident memory "sensing-and-alarm" function and ∼42 hours earlier than in lung infection by influenza virus. These data demonstrate relevance for Tem in protection against malaria and provide generalizable mechanistic insights germane to control of liver infections.

Sepsis leads to lasting changes in phenotype and function of memory CD8 T cells.

The global health burden due to sepsis and the associated cytokine storm is substantial. While early intervention has improved survival during the cytokine storm, those that survive can enter a state of chronic immunoparalysis defined by transient lymphopenia and functional deficits of surviving cells. Memory CD8 T cells provide rapid cytolysis and cytokine production following re-encounter with their cognate antigen to promote long-term immunity, and CD8 T cell impairment due to sepsis can pre-dispose individuals to re-infection. While the acute influence of sepsis on memory CD8 T cells has been characterized, if and to what extent pre-existing memory CD8 T cells recover remains unknown. Here, we observed that central memory CD8 T cells (TCM) from septic patients proliferate more than those from healthy individuals. Utilizing LCMV immune mice and a CLP model to induce sepsis, we demonstrated that TCM proliferation is associated with numerical recovery of pathogen-specific memory CD8 T cells following sepsis-induced lymphopenia. This increased proliferation leads to changes in composition of memory CD8 T cell compartment and altered tissue localization. Further, memory CD8 T cells from sepsis survivors have an altered transcriptional profile and chromatin accessibility indicating long-lasting T cell intrinsic changes. The sepsis-induced changes in the composition of the memory CD8 T cell pool and transcriptional landscape culminated in altered T cell function and reduced capacity to control L. monocytogenes infection. Thus, sepsis leads to long-term alterations in memory CD8 T cell phenotype, protective function and localization potentially changing host capacity to respond to re-infection.

A dirty cut, a nasty burn, a severe COVID infection; there are many ways for someone to develop sepsis. This life-threatening condition emerges when the immune system overreacts to a threat and ends up damaging the body. Even when patients survive, they are often left with a partially impaired immune system that cannot adequately protect against microbes and cancer; this is known as immunoparalysis. Memory CD8 T cells, a type of immune cell that is compromised by sepsis, are a long-lived population of cells that 'remember' previous infection or vaccination, and then react faster to prevent the same illness if the person ever encounters the same threat again. Yet it is unclear how exactly sepsis harms the function and representation of memory CD8 T cells, and the immune system in general. Jensen et al. investigated this question, first by showing that sepsis leads to a profound loss of memory CD8 T cells, but that surviving memory CD8 T cells multiply quickly ­ especially a subpopulation known as central memory CD8 T cells ­ to re-establish the memory CD8 T cell population. Since the central memory CD8 T cells proliferate better than the other memory T cells this alters the overall composition of the pool of memory CD8 T cells, with central memory cells becoming overrepresented. Further experiments revealed that this biasing toward central memory T cells, due to sepsis, created long-term changes in the distribution of memory CD8 T cells throughout the body. The way the genetic information of these cells was packaged had also been altered, as well as which genes were switched on or off. Overall, these changes reduced the ability of memory CD8 T cells to control infections. Together, these findings help to understand how immunoparalysis can emerge after sepsis, and what could be done to correct it. These findings could also be applied to other conditions ­ such as COVID-19 ­ which may cause similar long-term changes to the immune system.