T cells in the brain enhance neonatal mortality during peripheral LCMV infection.

In adult mice the severity of disease from viral infections is determined by the balance between the efficiency of the immune response and the magnitude of viral load. Here, the impact of this dynamic is examined in neonates. Newborns are highly susceptible to infections due to poor innate responses, lower numbers of T cells and Th2-prone immune responses. Eighty-percent of 7-day old mice, immunologically equivalent to human neonates, succumbed to extremely low doses (5 PFU) of the essentially non-lethal lymphocytic choriomeningitis virus (LCMV-Armstrong) given intraperitoneally. This increased lethality was determined to be dependent upon poor early viral control, as well as, T cells and perforin as assessed in knockout mice. By day 3, these neonatal mice had 400-fold higher viral loads as compared to adults receiving a 10,000-fold (5X104 PFU) higher dose of LCMV. The high viral load in combination with the subsequent immunological defect of partial CD8 T cell clonal exhaustion in the periphery led to viral entry and replication in the brain. Within the brain, CD8 T cells were protected from exhaustion, and thus were able to mediate lethal immunopathology. To further delineate the role of early viral control, neonatal mice were infected with Pichinde virus, a less virulent arenavirus, or LCMV was given to pups of LCMV-immune mothers. In both cases, peak viral load was at least 29-fold lower, leading to functional CD8 T cell responses and 100% survival.

SwarmTCR: a computational approach to predict the specificity of T cell receptors.

BACKGROUND: With more T cell receptor sequence data becoming available, the need for bioinformatics approaches to predict T cell receptor specificity is even more pressing. Here we present SwarmTCR, a method that uses labeled sequence data to predict the specificity of T cell receptors using a nearest-neighbor approach. SwarmTCR works by optimizing the weights of the individual CDR regions to maximize classification performance. RESULTS: We compared the performance of SwarmTCR against another nearest-neighbor method and showed that SwarmTCR performs well both with bulk sequencing data and with single cell data. In addition, we show that the weights returned by SwarmTCR are biologically interpretable. CONCLUSIONS: Computationally predicting the specificity of T cell receptors can be a powerful tool to shed light on the immune response against infectious diseases and cancers, autoimmunity, cancer immunotherapy, and immunopathology. SwarmTCR is distributed freely under the terms of the GPL-3 license. The source code and all sequencing data are available at GitHub ( https://github.com/thecodingdoc/SwarmTCR ).

CDR3α drives selection of the immunodominant Epstein Barr virus (EBV) BRLF1-specific CD8 T cell receptor repertoire in primary infection.

The T cell receptor (TCR) repertoire is an essential component of the CD8 T-cell immune response. Here, we seek to investigate factors that drive selection of TCR repertoires specific to the HLA-A2-restricted immunodominant epitope BRLF1109-117 (YVLDHLIVV) over the course of primary Epstein Barr virus (EBV) infection. Using single-cell paired TCRαß sequencing of tetramer sorted CD8 T cells ex vivo, we show at the clonal level that recognition of the HLA-A2-restricted BRLF1 (YVL-BR, BRLF-1109) epitope is mainly driven by the TCRα chain. For the first time, we identify a CDR3α (complementarity determining region 3 α) motif, KDTDKL, resulting from an obligate AV8.1-AJ34 pairing that was shared by all four individuals studied. This observation coupled with the fact that this public AV8.1-KDTDKL-AJ34 TCR pairs with multiple different TCRß chains within the same donor (median 4; range: 1-9), suggests that there are some unique structural features of the interaction between the YVL-BR/MHC and the AV8.1-KDTDKL-AJ34 TCR that leads to this high level of selection. Newly developed TCR motif algorithms identified a lysine at position 1 of the CDR3α motif that is highly conserved and likely important for antigen recognition. Crystal structure analysis of the YVL-BR/HLA-A2 complex revealed that the MHC-bound peptide bulges at position 4, exposing a negatively charged aspartic acid that may interact with the positively charged lysine of CDR3α. TCR cloning and site-directed mutagenesis of the CDR3α lysine ablated YVL-BR-tetramer staining and substantially reduced CD69 upregulation on TCR mutant-transduced cells following antigen-specific stimulation. Reduced activation of T cells expressing this CDR3 motif was also observed following exposure to mutated (D4A) peptide. In summary, we show that a highly public TCR repertoire to an immunodominant epitope of a common human virus is almost completely selected on the basis of CDR3α and provide a likely structural basis for the selection. These studies emphasize the importance of examining TCRα, as well as TCRß, in understanding the CD8 T cell receptor repertoire.

Transient expression of ZBTB32 in anti-viral CD8+ T cells limits the magnitude of the effector response and the generation of memory.

Virus infections induce CD8+ T cell responses comprised of a large population of terminal effector cells and a smaller subset of long-lived memory cells. The transcription factors regulating the relative expansion versus the long-term survival potential of anti-viral CD8+ T cells are not completely understood. We identified ZBTB32 as a transcription factor that is transiently expressed in effector CD8+ T cells. After acute virus infection, CD8+ T cells deficient in ZBTB32 showed enhanced virus-specific CD8+ T cell responses, and generated increased numbers of virus-specific memory cells; in contrast, persistent expression of ZBTB32 suppressed memory cell formation. The dysregulation of CD8+ T cell responses in the absence of ZBTB32 was catastrophic, as Zbtb32-/- mice succumbed to a systemic viral infection and showed evidence of severe lung pathology. We found that ZBTB32 and Blimp-1 were co-expressed following CD8+ T cell activation, bound to each other, and cooperatively regulated Blimp-1 target genes Eomes and Cd27. These findings demonstrate that ZBTB32 is a key transcription factor in CD8+ effector T cells that is required for the balanced regulation of effector versus memory responses to infection.

Heterologous Immunity and Persistent Murine Cytomegalovirus Infection.

One's history of infections can affect the immune response to unrelated pathogens and influence disease outcome through the process of heterologous immunity. This can occur after acute viral infections, such as infections with lymphocytic choriomeningitis virus (LCMV) and vaccinia virus, where the pathogens are cleared, but it becomes a more complex issue in the context of persistent infections. In this study, murine cytomegalovirus (MCMV) was used as a persistent infection model to study heterologous immunity with LCMV. If mice were previously immune to LCMV and then infected with MCMV (LCMV+MCMV), they had more severe immunopathology, enhanced viral burden in multiple organs, and suppression of MCMV-specific T cell memory inflation. MCMV infection initially reduced the numbers of LCMV-specific memory T cells, but continued MCMV persistence did not further erode memory T cells specific to LCMV. When MCMV infection was given first (MCMV+LCMV), the magnitude of the acute T cell response to LCMV declined with age though this age-dependent decline was not dependent on MCMV. However, some of these MCMV persistently infected mice with acute LCMV infection (7 of 36) developed a robust immunodominant CD8 T cell response apparently cross-reactive between a newly defined putative MCMV epitope sequence, M57727-734, and the normally subdominant LCMV epitope L2062-2069, indicating a profound private specificity effect in heterologous immunity between these two viruses. These results further illustrate how a history of an acute or a persistent virus infection can substantially influence the immune responses and immune pathology associated with acute or persistent infections with an unrelated virus. IMPORTANCE: This study extends our understanding of heterologous immunity in the context of persistent viral infection. The phenomenon has been studied mostly with viruses such as LCMV that are cleared, but the situation can be more complex with a persistent virus such as MCMV. We found that the history of LCMV infection intensifies MCMV immunopathology, enhances MCMV burden in multiple organs, and suppresses MCMV-specific T cell memory inflation. In the reverse infection sequence, we show that some of the long-term MCMV-immune mice mount a robust CD8 T cell cross-reactive response between a newly defined putative MCMV epitope sequence and a normally subdominant LCMV epitope. These results further illustrate how a history of infection can substantially influence the immune responses and immune pathology associated with infections with an unrelated virus.

Natural killer cells act as rheostats modulating antiviral T cells.

Antiviral T cells are thought to regulate whether hepatitis C virus (HCV) and human immunodeficiency virus (HIV) infections result in viral control, asymptomatic persistence or severe disease, although the reasons for these different outcomes remain unclear. Recent genetic evidence, however, has indicated a correlation between certain natural killer (NK)-cell receptors and progression of both HIV and HCV infection, implying that NK cells have a role in these T-cell-associated diseases. Although direct NK-cell-mediated lysis of virus-infected cells may contribute to antiviral defence during some virus infections--especially murine cytomegalovirus (MCMV) infections in mice and perhaps HIV in humans--NK cells have also been suspected of having immunoregulatory functions. For instance, NK cells may indirectly regulate T-cell responses by lysing MCMV-infected antigen-presenting cells. In contrast to MCMV, lymphocytic choriomeningitis virus (LCMV) infection in mice seems to be resistant to any direct antiviral effects of NK cells. Here we examine the roles of NK cells in regulating T-cell-dependent viral persistence and immunopathology in mice infected with LCMV, an established model for HIV and HCV infections in humans. We describe a three-way interaction, whereby activated NK cells cytolytically eliminate activated CD4 T cells that affect CD8 T-cell function and exhaustion. At high virus doses, NK cells prevented fatal pathology while enabling T-cell exhaustion and viral persistence, but at medium doses NK cells paradoxically facilitated lethal T-cell-mediated pathology. Thus, NK cells can act as rheostats, regulating CD4 T-cell-mediated support for the antiviral CD8 T cells that control viral pathogenesis and persistence.

Regulatory T cells resist virus infection-induced apoptosis.

UNLABELLED: Regulatory T (Treg) cells are important in the maintenance of self-tolerance, and the depletion of Treg cells correlates with autoimmune development. It has been shown that type I interferon (IFN) responses induced early in the infection of mice can drive memory (CD44hi) CD8 and CD4 T cells into apoptosis, and we questioned here whether the apoptosis of CD44-expressing Treg cells might be involved in the infection-associated autoimmune development. Instead, we found that Treg cells were much more resistant to apoptosis than CD44hi CD8 and CD4 T cells at days 2 to 3 after lymphocytic choriomeningitis virus infection, when type I IFN levels are high. The infection caused a downregulation of the interleukin-7 (IL-7) receptor, needed for survival of conventional T cells, while increasing on Treg cells the expression of the high-affinity IL-2 receptor, needed for STAT5-dependent survival of Treg cells. The stably maintained Treg cells early during infection may explain the relatively low incidence of autoimmune manifestations among infected patients. IMPORTANCE: Autoimmune diseases are controlled in part by regulatory T cells (Treg) and are thought to sometimes be initiated by viral infections. We tested the hypothesis that Treg may die off at early stages of infection, when virus-induced factors kill other lymphocyte types. Instead, we found that Treg resisted this cell death, perhaps reducing the tendency of viral infections to cause immune dysfunction and induce autoimmunity.

Narrowing of human influenza A virus-specific T cell receptor α and ß repertoires with increasing age.

UNLABELLED: Alterations in memory CD8 T cell responses may contribute to the high morbidity and mortality caused by seasonal influenza A virus (IAV) infections in older individuals. We questioned whether memory CD8 responses to this nonpersistent virus, to which recurrent exposure with new strains is common, changed over time with increasing age. Here, we show a direct correlation between increasing age and narrowing of the HLA-A2-restricted IAV Vα and Vß T cell repertoires specific to M1 residues 58 to 66 (M158-66), which simultaneously lead to oligoclonal expansions, including the usage of a single identical VA12-JA29 clonotype in all eight older donors. The Vα repertoire of older individuals also had longer CDR3 regions with increased usage of G/A runs, whose molecular flexibility may enhance T cell receptor (TCR) promiscuity. Collectively, these results suggest that CD8 memory T cell responses to nonpersistent viruses like IAV in humans are dynamic, and with aging there is a reduced diversity but a preferential retention of T cell repertoires with features of enhanced cross-reactivity. IMPORTANCE: With increasing age, the immune system undergoes drastic changes, and older individuals have declined resistance to infections. Vaccinations become less effective, and infection with influenza A virus in older individuals is associated with higher morbidity and mortality. Here, we questioned whether T cell responses directed against the highly conserved HLA-A2-restricted M158-66 peptide of IAV evolves with increasing age. Specifically, we postulated that CD8 T cell repertoires narrow with recurrent exposure and may thus be less efficient in response to new infections with new strains of IAV. Detailed analyses of the VA and VB TCR repertoires simultaneously showed a direct correlation between increasing age and narrowing of the TCR repertoire. Features of the TCRs indicated potentially enhanced cross-reactivity in all older donors. In summary, T cell repertoire analysis in older individuals may be useful as one of the predictors of protection after vaccination.

Increased Immune Response Variability during Simultaneous Viral Coinfection Leads to Unpredictability in CD8 T Cell Immunity and Pathogenesis.

UNLABELLED: T cell memory is usually studied in the context of infection with a single pathogen in naive mice, but how memory develops during a coinfection with two pathogens, as frequently occurs in nature or after vaccination, is far less studied. Here, we questioned how the competition between immune responses to two viruses in the same naive host would influence the development of CD8 T cell memory and subsequent disease outcome upon challenge. Using two different models of coinfection, including the well-studied lymphocytic choriomeningitis (LCMV) and Pichinde (PICV) viruses, several differences were observed within the CD8 T cell responses to either virus. Compared to single-virus infection, coinfection resulted in substantial variation among mice in the size of epitope-specific T cell responses to each virus. Some mice had an overall reduced number of virus-specific cells to either one of the viruses, and other mice developed an immunodominant response to a normally subdominant, cross-reactive epitope (nucleoprotein residues 205 to 212, or NP205). These changes led to decreased protective immunity and enhanced pathology in some mice upon challenge with either of the original coinfecting viruses. In mice with PICV-dominant responses, during a high-dose challenge with LCMV clone 13, increased immunopathology was associated with a reduced number of LCMV-specific effector memory CD8 T cells. In mice with dominant cross-reactive memory responses, during challenge with PICV increased immunopathology was directly associated with these cross-reactive NP205-specific CD8 memory cells. In conclusion, the inherent competition between two simultaneous immune responses results in significant alterations in T cell immunity and subsequent disease outcome upon reexposure. IMPORTANCE: Combination vaccines and simultaneous administration of vaccines are necessary to accommodate required immunizations and maintain vaccination rates. Antibody responses generally correlate with protection and vaccine efficacy. However, live attenuated vaccines also induce strong CD8 T cell responses, and the impact of these cells on subsequent immunity, whether beneficial or detrimental, has seldom been studied, in part due to the lack of known T cell epitopes to vaccine viruses. We questioned if the inherent increased competition and stochasticity between two immune responses during a simultaneous coinfection would significantly alter CD8 T cell memory in a mouse model where CD8 T cell epitopes are clearly defined. We show that some of the coinfected mice have sufficiently altered memory T cell responses that they have decreased protection and enhanced immunopathology when reexposed to one of the two viruses. These data suggest that a better understanding of human T cell responses to vaccines is needed to optimize immunization strategies.

A small jab - a big effect: nonspecific immunomodulation by vaccines.

Recent epidemiological studies have shown that, in addition to disease-specific effects, vaccines against infectious diseases have nonspecific effects on the ability of the immune system to handle other pathogens. For instance, in randomized trials tuberculosis and measles vaccines are associated with a substantial reduction in overall child mortality, which cannot be explained by prevention of the target disease. New research suggests that the nonspecific effects of vaccines are related to cross-reactivity of the adaptive immune system with unrelated pathogens, and to training of the innate immune system through epigenetic reprogramming. Hence, epidemiological findings are backed by immunological data. This generates a new understanding of the immune system and about how it can be modulated by vaccines to impact the general resistance to disease.

Innate PLZF+CD4+ αß T cells develop and expand in the absence of Itk.

T cell development in the thymus produces multiple lineages of cells, including innate T cells. Studies in mice harboring alterations in TCR signaling proteins or transcriptional regulators have revealed an expanded population of CD4(+) innate T cells in the thymus that produce IL-4 and express the transcription factor promyelocytic leukemia zinc finger (PLZF). In these mice, IL-4 produced by the CD4(+)PLZF(+) T cell population leads to the conversion of conventional CD8(+) thymocytes into innate CD8(+) T cells resembling memory T cells expressing eomesodermin. The expression of PLZF, the signature invariant NKT cell transcription factor, in these innate CD4(+) T cells suggests that they might be a subset of αß or γδ TCR(+) NKT cells or mucosal-associated invariant T (MAIT) cells. To address these possibilities, we characterized the CD4(+)PLZF(+) innate T cells in itk(-/-) mice. We show that itk(-/-) innate PLZF(+)CD4(+) T cells are not CD1d-dependent NKT cells, MR1-dependent MAIT cells, or γδ T cells. Furthermore, although the itk(-/-) innate PLZF(+)CD4(+) T cells express αß TCRs, neither ß2-microglobulin-dependent MHC class I nor any MHC class II molecules are required for their development. In contrast to invariant NKT cells and MAIT cells, this population has a highly diverse TCRα-chain repertoire. Analysis of peripheral tissues indicates that itk(-/-) innate PLZF(+)CD4(+) T cells preferentially home to spleen and mesenteric lymph nodes owing to increased expression of gut-homing receptors, and that their expansion is regulated by commensal gut flora. These data support the conclusion that itk(-/-) innate PLZF(+)CD4(+) T cells are a novel subset of innate T cells.

Anti-IFN-γ and peptide-tolerization therapies inhibit acute lung injury induced by cross-reactive influenza A-specific memory T cells.

Viral infections have variable outcomes, with severe disease occurring in only few individuals. We hypothesized that this variable outcome could correlate with the nature of responses made to previous microbes. To test this, mice were infected initially with influenza A virus (IAV) and in memory phase challenged with lymphocytic choriomeningitis virus (LCMV), which we show in this study to have relatively minor cross-reactivity with IAV. The outcome in genetically identical mice varied from mild pneumonitis to severe acute lung injury with extensive pneumonia and bronchiolization, similar to that observed in patients who died of the 1918 H1N1 pandemic. Lesion expression did not correlate with virus titers. Instead, disease severity directly correlated with and was predicted by the frequency of IAV-PB1703- and IAV-PA224-specific responses, which cross-reacted with LCMV-GP34 and LCMV-GP276, respectively. Eradication or functional ablation of these pathogenic memory T cell populations, using mutant-viral strains, peptide-based tolerization strategies, or short-term anti-IFN-γ treatment, inhibited severe lesions such as bronchiolization from occurring. Heterologous immunity can shape outcome of infections and likely individual responses to vaccination, and can be manipulated to treat or prevent severe pathology.

Attrition of memory CD8 T cells.

An important role for the immune system is to maintain protective immunological memory to a wide variety of pathogens encountered over one's lifetime, while still leaving the host able to respond to newly encountered pathogens. Vezys et al. make the interesting observation that it is possible to repeatedly immunize mice in ways that allow for development of high numbers of memory CD8 T cells without depleting pre-existing memory cells specific for other pathogens. This study, which offers promise in developing potent vaccination schemes, is seemingly at odds with work published by us in the 1990s showing a loss in CD8 memory cells after a series of infections. In their reply, Vezys et al. mention that we may have misinterpreted our data because we reported the putative loss of memory T cells as per cent rather than total number, but here we represent the data in those studies as total cell number. We show here in Fig. 1 that a series of infections can indeed reduce the total number of memory cells, indicating that vaccination strategies need to consider this issue.

No one is naive: the significance of heterologous T-cell immunity.

Memory T cells that are specific for one virus can become activated during infection with an unrelated heterologous virus, and might have roles in protective immunity and immunopathology. The course of each infection is influenced by the T-cell memory pool that has been laid down by a host's history of previous infections, and with each successive infection, T-cell memory to previously encountered agents is modified. Here, we discuss evidence from studies in mice and humans that shows the importance of this phenomenon in determining the outcome of infection.

PC61 (anti-CD25) treatment inhibits influenza A virus-expanded regulatory T cells and severe lung pathology during a subsequent heterologous lymphocytic choriomeningitis virus infection.

Prior immunity to influenza A virus (IAV) in mice changes the outcome to a subsequent lymphocytic choriomeningitis virus (LCMV) infection and can result in severe lung pathology, similar to that observed in patients that died of the 1918 H1N1 pandemic. This pathology is induced by IAV-specific memory CD8(+) T cells cross-reactive with LCMV. Here, we discovered that IAV-immune mice have enhanced CD4(+) Foxp3(+) T-regulatory (Treg) cells in their lungs, leading us to question whether a modulation in the normal balance of Treg and effector T-cell responses also contributes to enhancing lung pathology upon LCMV infection of IAV-immune mice. Treg cell and interleukin-10 (IL-10) levels remained elevated in the lungs and mediastinal lymph nodes (mLNs) throughout the acute LCMV response of IAV-immune mice. PC61 treatment, used to decrease Treg cell levels, did not change LCMV titers but resulted in a surprising decrease in lung pathology upon LCMV infection in IAV-immune but not in naive mice. Associated with this decrease in pathology was a retention of Treg in the mLN and an unexpected partial clonal exhaustion of LCMV-specific CD8(+) T-cell responses only in IAV-immune mice. PC61 treatment did not affect cross-reactive memory CD8(+) T-cell proliferation. These results suggest that in the absence of IAV-expanded Treg cells and in the presence of cross-reactive memory, the LCMV-specific response was overstimulated and became partially exhausted, resulting in a decreased effector response. These studies suggest that Treg cells generated during past infections can influence the characteristics of effector T-cell responses and immunopathology during subsequent heterologous infections. Thus, in humans with complex infection histories, PC61 treatment may lead to unexpected results.

Loss of anti-viral immunity by infection with a virus encoding a cross-reactive pathogenic epitope.

T cell cross-reactivity between different strains of the same virus, between different members of the same virus group, and even between unrelated viruses is a common occurrence. We questioned here how an intervening infection with a virus containing a sub-dominant cross-reactive T cell epitope would affect protective immunity to a previously encountered virus. Pichinde virus (PV) and lymphocytic choriomeningitis virus (LCMV) encode subdominant cross-reactive NP205₋212 CD8 T cell epitopes sharing 6 of 8 amino acids, differing only in the MHC anchoring regions. These pMHC epitopes induce cross-reactive but non-identical T cell receptor (TCR) repertoires, and structural studies showed that the differing anchoring amino acids altered the conformation of the MHC landscape presented to the TCR. PV-immune mice receiving an intervening infection with wild type but not NP205-mutant LCMV developed severe immunopathology in the form of acute fatty necrosis on re-challenge with PV, and this pathology could be predicted by the ratio of NP205-specific to the normally immunodominant PV NP38₋45-specific T cells. Thus, cross-reactive epitopes can exert pathogenic properties that compromise protective immunity by impairing more protective T cell responses.

Heterologous immunity between viruses.

Immune memory responses to previously encountered pathogens can sometimes alter the immune response to and the course of infection of an unrelated pathogen by a process known as heterologous immunity. This response can lead to enhanced or diminished protective immunity and altered immunopathology. Here, we discuss the nature of T-cell cross-reactivity and describe matrices of epitopes from different viruses eliciting cross-reactive CD8(+) T-cell responses. We examine the parameters of heterologous immunity mediated by these cross-reactive T cells during viral infections in mice and humans. We show that heterologous immunity can disrupt T-cell memory pools, alter the complexity of the T-cell repertoire, change patterns of T-cell immunodominance, lead to the selection of viral epitope-escape variants, alter the pathogenesis of viral infections, and, by virtue of the private specificity of T-cell repertoires within individuals, contribute to dramatic variations in viral disease. We propose that heterologous immunity is an important factor in resistance to and variations of human viral infections and that issues of heterologous immunity should be considered in the design of vaccines.

Identification of CD8 T-cell dysfunction associated with symptoms in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID and treatment with a nebulized antioxidant/anti-pathogen agent in a retrospective case series.

Background: Patients with post-acute sequelae of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection (PASC, i.e., Long COVID) have a symptom complex highly analogous to many features of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), suggesting they may share some aspects of pathogenesis in these similar disorders. ME/CFS is a complex disease affecting numerous organ systems and biological processes and is often preceded by an infection-like episode. It is postulated that the chronic manifestations of illness may result from an altered host response to infection or inability to resolve inflammation, as is being reported in Long COVID. The immunopathogenesis of both disorders is still poorly understood. Here, we show data that suggest Long COVID and ME/CFS may be due to an aberrant response to an immunological trigger-like infection, resulting in a dysregulated immune system with CD8 T-cell dysfunction reminiscent of some aspects of T-cell clonal exhaustion, a phenomenon associated with oxidative stress. As there is an urgent need for diagnostic tools and treatment strategies for these two related disabling disorders, here, in a retrospective case series, we have also identified a potential nebulized antioxidant/anti-pathogen treatment that has evidence of a good safety profile. This nebulized agent is comprised of five ingredients previously reported individually to relieve oxidative stress, attenuate NF-κB signaling, and/or to act directly to inhibit pathogens, including viruses. Administration of this treatment by nebulizer results in rapid access of small doses of well-studied antioxidants and agents with anti-pathogen potential to the lungs; components of this nebulized agent are also likely to be distributed systemically, with potential to enter the central nervous system. Methods: and Findings: We conducted an analysis of CD8 T-cell function and severity of symptoms by self-report questionnaires in ME/CFS, Long COVID and healthy controls. We developed a CD8 T-cell functional assay, assessing CD8 T-cell dysfunction by intracellular cytokine staining (ICS) in a group of ME/CFS (n = 12) and Long COVID patients (n = 8), comparing to healthy controls (HC) with similar age and sex (n = 10). Magnet-enriched fresh CD8 T-cells in both patient groups had a significantly diminished capacity to produce both cytokines, IFNγ or TNFα, after PMA stimulation when compared to HC. The symptom severity questionnaire showed similar symptom profiles for the two disorders. Fortuitously, through a retrospective case series, we were able to examine the ICS and questionnaire data of 4 ME/CFS and 4 Long COVID patients in conjunction with their treatment (3-15 months). In parallel with the treatment pursued electively by participants in this retrospective case series, there was an increase in CD8 T-cell IFNγ and TNFα production and a decrease in overall self-reported symptom severity score by 54%. No serious treatment-associated side effects or laboratory anomalies were noted in these patients. Conclusions: Here, in this small study, we present two observations that appear potentially fundamental to the pathogenesis and treatment of Long COVID and ME/CFS. The first is that both disorders appear to be characterized by dysfunctional CD8 T-cells with severe deficiencies in their abilities to produce IFNγ and TNFα. The second is that in a small retrospective Long COVID and ME/CFS case series, this immune dysfunction and patient health improved in parallel with treatment with an immunomodulatory, antioxidant pharmacological treatment with anticipated anti-pathogen activity. This work provides evidence of the potential utility of a biomarker, CD8 T-cell dysfunction, and suggests the potential for benefit from a new nebulized antioxidant/anti-pathogen treatment. These immune biomarker data may help build capacity for improved diagnosis and tracking of treatment outcomes during clinical trials for both Long COVID and ME/CFS while providing clues to new treatment avenues that suggest potential efficacy for both conditions.