Combination immunotherapy with anti-PD-L1 antibody and depletion of regulatory T cells during acute viral infections results in improved virus control but lethal immunopathology.

Combination immunotherapy (CIT) is currently applied as a treatment for different cancers and is proposed as a cure strategy for chronic viral infections. Whether such therapies are efficient during an acute infection remains elusive. To address this, inhibitory receptors were blocked and regulatory T cells depleted in acutely Friend retrovirus-infected mice. CIT resulted in a dramatic expansion of cytotoxic CD4+ and CD8+ T cells and a subsequent reduction in viral loads. Despite limited viral replication, mice developed fatal immunopathology after CIT. The pathology was most severe in the gastrointestinal tract and was mediated by granzyme B producing CD4+ and CD8+ T cells. A similar post-CIT pathology during acute Influenza virus infection of mice was observed, which could be prevented by vaccination. Melanoma patients who developed immune-related adverse events under immune checkpoint CIT also presented with expanded granzyme-expressing CD4+ and CD8+ T cell populations. Our data suggest that acute infections may induce immunopathology in patients treated with CIT, and that effective measures for infection prevention should be applied.

Critical role of PD-L1 expression on non-tumor cells rather than on tumor cells for effective anti-PD-L1 immunotherapy in a transplantable mouse hematopoietic tumor model.

The expression of PD-L1 on tumor cells or within the tumor microenvironment has been associated with good prognosis and sustained clinical responses in immunotherapeutic regimens based on PD-L1/PD-1/CD80 immune checkpoint blockade. To look into the current controversy in cancer immunotherapy of the relative importance of PD-L1 expression on tumor cells versus non-tumor cells of the tumor microenvironment, a hematological mouse tumor model was chosen. By combining a genetic CRISPR/Cas9 and immunotherapeutic approach and using a syngeneic hematopoietic transplantable tumor model (E.G7-cOVA tumor cells), we demonstrated that dual blockade of PD-L1 interaction with PD-1 and CD80 enhanced anti-tumor immune responses that either delayed tumor growth or led to its complete eradication. PD-L1 expression on non-tumor cells of the tumor microenvironment was required for the promotion of tumor immune escape and its blockade elicited potent anti-tumor responses to PD-L1 WT and to PD-L1-deficient tumor cells. PD-L1+ tumors implanted in PD-L1-deficient mice exhibited delayed tumor growth independently of PD-L1 blockade. These findings emphasize that PD-L1 expression on non-tumor cells plays a major role in this tumor model. These observations should turn our attention to the tumor microenvironment in hematological malignancies because of its unappreciated contribution to create a conditioned niche for the tumor to grow and evade the anti-tumor immune response.

MDSCs in infectious diseases: regulation, roles, and readjustment.

Many pathogens, ranging from viruses to multicellular parasites, promote expansion of MDSCs, which are myeloid cells that exhibit immunosuppressive features. The roles of MDSCs in infection depend on the class and virulence mechanisms of the pathogen, the stage of the disease, and the pathology associated with the infection. This work compiles evidence supported by functional assays on the roles of different subsets of MDSCs in acute and chronic infections, including pathogen-associated malignancies, and discusses strategies to modulate MDSC dynamics to benefit the host.

Differential Inhibitory Receptor Expression on T Cells Delineates Functional Capacities in Chronic Viral Infection.

Inhibitory receptors have been extensively described for their importance in regulating immune responses in chronic infections and cancers. Blocking the function of inhibitory receptors such as PD-1, CTLA-4, 2B4, Tim-3, and LAG-3 has shown promise for augmenting CD8 T cell activity and boosting pathogen-specific immunity. However, the prevalence of inhibitory receptors on CD4 T cells and their relative influence on CD4 T cell functionality in chronic HIV infection remains poorly described. We therefore determined and compared inhibitory receptor expression patterns of 2B4, CTLA-4, LAG-3, PD-1, and Tim-3 on virus-specific CD4 and CD8 T cells in relation to their functional T cell profile. In chronic HIV infection, inhibitory receptor distribution differed markedly between cytokine-producing T cell subsets with, gamma interferon (IFN-γ)- and tumor necrosis factor alpha (TNF-α)-producing cells displaying the highest and lowest prevalence of inhibitory receptors, respectively. Blockade of inhibitory receptors differentially affected cytokine production by cells in response to staphylococcal enterotoxin B stimulation. CTLA-4 blockade increased IFN-γ and CD40L production, while PD-1 blockade strongly augmented IFN-γ, interleukin-2 (IL-2), and TNF-α production. In a Friend retrovirus infection model, CTLA-4 blockade in particular was able to improve control of viral replication. Together, these results show that inhibitory receptor distribution on HIV-specific CD4 T cells varies markedly with respect to the functional subset of CD4 T cells being analyzed. Furthermore, the differential effects of receptor blockade suggest novel methods of immune response modulation, which could be important in the context of HIV vaccination or therapeutic strategies.IMPORTANCE Inhibitory receptors are important for limiting damage by the immune system during acute infections. In chronic infections, however, their expression limits immune system responsiveness. Studies have shown that blocking inhibitory receptors augments CD8 T cell functionality in HIV infection, but their influence on CD4 T cells remains unclear. We assessed the expression of inhibitory receptors on HIV-specific CD4 T cells and their relationship with T cell functionality. We uncovered differences in inhibitory receptor expression depending on the CD4 T cell function. We also found differences in functionality of CD4 T cells following blocking of different inhibitory receptors, and we confirmed our results in a Friend virus retroviral model of infection in mice. Our results show that inhibitory receptor expression on CD4 T cells is linked to CD4 T cell functionality and could be sculpted by blockade of specific inhibitory receptors. These data reveal exciting possibilities for the development of novel treatments and immunotherapeutics.

CD137 Agonist Therapy Can Reprogram Regulatory T Cells into Cytotoxic CD4+ T Cells with Antitumor Activity.

Recent successes in immune therapeutic strategies aimed to improve control over tumor growth have sparked hope that long-lived control of cancer through stimulation of the immune system can be possible. However, the underlying immunological mechanisms that are induced by immunotherapeutic strategies are not well understood. In this study, we used the highly immunogenic Friend virus-induced FBL-3 tumor as a model to study the mechanisms of immunological tumor control by CD4(+) T cells in the course of CD137 (4-1BB) agonist immunotherapy in the absence of a CD8 T cell response. We demonstrate that treatment with a CD137 agonist resulted in complete FBL-3 tumor regression in CD8(+) T cell-deficient mice. CD137 signaling enhanced the production of proinflammatory cytokines and cytotoxic molecules in tumor-specific CD4(+) T cells. Interestingly, a subset of CD4(+)Foxp3(+) regulatory T cells was reprogrammed to eliminate immunogenic virus-induced tumor cells in response to CD137 agonist treatment. These cells expressed markers characteristic for Th cells (CD154) and produced the cytokine TNF-α or the T-box transcriptional factor Eomesodermin and granzyme B without loss of Foxp3 expression. Foxp3 Eomes double-positive CD4(+) T cells were capable of eliminating immunogenic virus-induced tumor cells in vivo. Thus, our data show that tumor-induced Foxp3(+)CD4(+) T cells can be reprogrammed into cytotoxic effector cells upon therapeutic costimulatory signaling and restore antitumor immunity.

Granulocytic myeloid-derived suppressor cells suppress virus-specific CD8+ T cell responses during acute Friend retrovirus infection.

BACKGROUND: Myeloid-derived suppressor cells (MDSCs) can suppress T cell responses in several different diseases. Previously these suppressive cells were observed to expand in HIV patients and in a mouse retrovirus model, yet their suppressive effect on virus-specific CD8+ T cells in vitro and in vivo has not been characterized thus far. RESULTS: We used the Friend retrovirus (FV) model to demonstrate that MDSCs expand and become activated during the late phase of acute FV infection. Only the subpopulation of granulocytic MDSCs (gMDSCs) but not monocytic MDSC suppressed virus-specific CD8+ T cell proliferation and function in vitro. gMDSCs expressed arginase 1, high levels of the inhibitory ligand PD-L1 and the ATP dephosphorylating enzyme CD39 on the cell surface upon infection. All three molecules were involved in the suppressive effect of the gMDSCs in vitro. MDSC depletion experiments in FV-infected mice revealed that they restrict virus-specific CD8+ T cell responses and thus affect the immune control of chronic retroviruses in vivo. CONCLUSIONS: Our study demonstrates that MDSCs become activated and expand during the acute phase of retrovirus infection. Their suppressive activity on virus-specific CD8+ T cells may contribute to T cell dysfunction and the development of chronic infection.

PD-L1 Expression on Retrovirus-Infected Cells Mediates Immune Escape from CD8+ T Cell Killing.

Cytotoxic CD8+ T Lymphocytes (CTL) efficiently control acute virus infections but can become exhausted when a chronic infection develops. Signaling of the inhibitory receptor PD-1 is an important mechanism for the development of virus-specific CD8+ T cell dysfunction. However, it has recently been shown that during the initial phase of infection virus-specific CD8+ T cells express high levels of PD-1, but are fully competent in producing cytokines and killing virus-infected target cells. To better understand the role of the PD-1 signaling pathway in CD8+ T cell cytotoxicity during acute viral infections we analyzed the expression of the ligand on retrovirus-infected cells targeted by CTLs. We observed increased levels of PD-L1 expression after infection of cells with the murine Friend retrovirus (FV) or with HIV. In FV infected mice, virus-specific CTLs efficiently eliminated infected target cells that expressed low levels of PD-L1 or that were deficient for PD-L1 but the population of PD-L1high cells escaped elimination and formed a reservoir for chronic FV replication. Infected cells with high PD-L1 expression mediated a negative feedback on CD8+ T cells and inhibited their expansion and cytotoxic functions. These findings provide evidence for a novel immune escape mechanism during acute retroviral infection based on PD-L1 expression levels on virus infected target cells.

Combining regulatory T cell depletion and inhibitory receptor blockade improves reactivation of exhausted virus-specific CD8+ T cells and efficiently reduces chronic retroviral loads.

Chronic infections with human viruses, such as HIV and HCV, or mouse viruses, such as LCMV or Friend Virus (FV), result in functional exhaustion of CD8(+) T cells. Two main mechanisms have been described that mediate this exhaustion: expression of inhibitory receptors on CD8(+) T cells and expansion of regulatory T cells (Tregs) that suppress CD8(+) T cell activity. Several studies show that blockage of one of these pathways results in reactivation of CD8(+) T cells and partial reduction in chronic viral loads. Using blocking antibodies against PD-1 ligand and Tim-3 and transgenic mice in which Tregs can be selectively ablated, we compared these two treatment strategies and combined them for the first time in a model of chronic retrovirus infection. Blocking inhibitory receptors was more efficient than transient depletion of Tregs in reactivating exhausted CD8(+) T cells and reducing viral set points. However, a combination therapy was superior to any single treatment and further augmented CD8(+) T cell responses and resulted in a sustained reduction in chronic viral loads. These results demonstrate that Tregs and inhibitory receptors are non-overlapping factors in the maintenance of chronic viral infections and that immunotherapies targeting both pathways may be a promising strategy to treat chronic infectious diseases.

TLR1/2 ligand-stimulated mouse liver endothelial cells secrete IL-12 and trigger CD8+ T cell immunity in vitro.

Liver sinusoidal endothelial cells (LSECs) are unique organ-resident APCs capable of Ag cross-presentation and subsequent tolerization of naive CD8(+) T cells. Under certain conditions, LSECs can switch from a tolerogenic to an immunogenic state and promote the development of T cell immunity. However, little is known about the mechanisms of LSECs to induce T cell immunity. In this study, we investigated whether functional maturation of LSECs can be achieved by TLR ligand stimulation and elucidated the mechanisms involved in LSEC-induced T cell immunity. We demonstrate that pretreatment of LSECs with palmitoyl-3-cysteine-serine-lysine-4 (P3C; TLR1/2 ligand) but not poly(I:C) (TLR3 ligand) or LPS (TLR4 ligand) reverted their suppressive properties to induce T cell immunity. Importantly, P3C stimulation caused functional maturation of Ag-presenting LSECs and enabled them to activate virus-specific CD8(+) T cells. The LSEC-mediated CD8(+) T cell immunity was initiated by soluble mediators, one of which was IL-12 secreted at a low but sustained level after P3C stimulation. P3C stimulation did not induce programmed death ligand 1 expression on LSECs, thereby favoring T cell proliferation and activation instead of suppression. Our data suggest that LSECs undergo maturation exclusively in response to TLR1/2 ligand stimulation and that the immunological status of LSECs was dependent upon the balance between programmed death ligand 1 and IL-12 expression. These results have implications for our understanding of liver-specific tolerance and autoimmunity and for the development of strategies to overcome T cell tolerance in situations such as chronic viral liver infections or liver cancer.

CD4+ T cells develop antiretroviral cytotoxic activity in the absence of regulatory T cells and CD8+ T cells.

Conventional CD4(+) T cells play an important role in viral immunity. In most virus infections, they provide essential help for antiviral B and T cell responses. In chronic infections, including HIV infection, an expansion of regulatory T cells (Tregs) has been demonstrated, which can suppress virus-specific CD4(+) T cell responses in vitro. However, the suppressive activity of Tregs on effector CD4(+) T cells in retroviral infection is less well documented in vivo. We took advantage of a transgenic mouse in which Tregs can be selectively depleted to determine the influence of such cells on retrovirus-specific CD4(+) T cell responses during an ongoing infection. Mice were infected with Friend retrovirus (FV), and Tregs were depleted during the acute phase of the infection. In nondepleted mice, activated CD4(+) T cells produced Th1-type cytokines but did not exhibit any antiviral cytotoxicity as determined in a major histocompatibility complex (MHC) class II-restricted in vivo cytotoxic T lymphocyte (CTL) assay. Depletion of Tregs significantly increased the numbers of virus-specific CD4(+) T cells and improved their cytokine production, whereas it induced only very little CD4(+) T cell cytotoxicity. However, after dual depletion of Tregs and CD8(+) T cells, conventional CD4(+) T cells developed significant cytotoxic activity against FV epitope-labeled target cells in vivo and contributed to the control of virus replication. Thus, both Tregs and CD8(+) T cells influence the cytotoxic activity of conventional CD4(+) T cells during an acute retroviral infection.

Transient depletion of regulatory T cells in transgenic mice reactivates virus-specific CD8+ T cells and reduces chronic retroviral set points.

Although chronic infections with viruses such as HIV and hepatitis C virus have been associated with regulatory T cell (Treg)-mediated suppression of virus-specific CD8(+) T-cell activity, no causal relationship between Tregs and chronic viral set points has been established. Using transgenic mice in which Tregs can be selectively ablated, we now show that transient depletion of Tregs during a chronic retroviral infection allows exhausted CD8(+) T cells to regain antiviral functions, including secretion of cytokines, production of cytotoxic molecules, and virus-specific cytolytic activity. Furthermore, short-term Treg ablation resulted in long-term reductions in chronic virus loads. These results demonstrate that Treg-mediated immunosuppression can be a significant factor in the maintenance of chronic viral infections and that Treg-targeted immunotherapy could be a valuable component in therapeutic strategies to treat chronic infectious diseases.

Natural regulatory T cells inhibit production of cytotoxic molecules in CD8⁺ T cells during low-level Friend retrovirus infection.

BACKGROUND: Cytotoxic T cells (CTL) play a central role in the control of viral infections. Their antiviral activity can be mediated by at least two cytotoxic pathways, namely the granule exocytosis pathway, involving perforin and granzymes, and the Fas-FasL pathway. It was shown that the level of Friend retrovirus (FV) replication determines the cytotoxic pathway for the control of viral infection. In low-level infection only the Fas pathway is active, whereas cytotoxic molecules are not produced. In the current study, we elucidate the role of CD4⁺ regulatory T cells (Tregs) in suppressing the exocytosis pathway during an asymptomatic low-level infection. FINDINGS: We show that even a low-level retrovirus infection induced a strong activation and proliferation of natural Tregs. The expanded Tregs suppressed the proliferation of virus-specific CD8⁺ T cells and the production of cytotoxic molecules by these cells. Not surprisingly, the in vivo killing activity of these CD8⁺ T cells was rather weak. Selective depletion of Foxp3⁺ Tregs resulted in de novo granzyme production and augmented virus-specific in vivo killing, but did not affect the low-level virus replication. CONCLUSIONS: Expanded natural Tregs determined the cytotoxic pathways of virus-specific effector CD8⁺ T cells during the acute phase of retroviral infection.

Tumor-specific CD4+ T cells develop cytotoxic activity and eliminate virus-induced tumor cells in the absence of regulatory T cells.

The important role of tumor-specific cytotoxic CD8(+) T cells is well defined in the immune control of the tumors, but the role of effector CD4(+) T cells is poorly understood. In the current research, we have used a murine retrovirus-induced tumor cell line of C57BL/6 mouse origin, namely FBL-3 cells, as a model to study basic mechanisms of immunological control and escape during tumor formation. This study shows that tumor-specific CD4(+) T cells are able to protect against virus-induced tumor cells. We show here that there is an expansion of tumor-specific CD4(+) T cells producing cytokines and cytotoxic molecule granzyme B (GzmB) in the early phase of tumor growth. Importantly, we demonstrate that in vivo depletion of regulatory T cells (Tregs) and CD8(+) T cells in FBL-3-bearing DEREG transgenic mice augments IL-2 and GzmB production by CD4(+) T cells and increases FV-specific CD4(+) T-cell effector and cytotoxic responses leading to the complete tumor regression. Therefore, the capacity to reject tumor acquired by tumor-reactive CD4(+) T cells largely depends on the direct suppressive activity of Tregs. We suggest that a cytotoxic CD4(+) T-cell immune response may be induced to enhance resistance against oncovirus-associated tumors.

Virus-specific CD8+ T cells upregulate programmed death-1 expression during acute friend retrovirus infection but are highly cytotoxic and control virus replication.

It was recently reported that inhibitory molecules such as programmed death-1 (PD-1) were upregulated on CD8(+) T cells during acute Friend retrovirus infection and that the cells were prematurely exhausted and dysfunctional in vitro. The current study confirms that most activated CD8(+) T cells upregulated expression of PD-1 during acute infection and revealed a dichotomy of function between PD-1(hi) and PD-1(lo) subsets. More PD-1(lo) cells produced antiviral cytokines such as IFN-γ and TNF-α, whereas more PD-1(hi) cells displayed characteristics of cytotoxic effectors such as production of granzymes and surface expression of CD107a. Importantly, CD8(+) T cells mediated rapid in vivo cytotoxicity and were critical for control of acute Friend virus replication. Thus, direct ex vivo analyses and in vivo experiments revealed high CD8(+) T cell functionality and indicate that PD-1 expression during acute infection is not a marker of T cell exhaustion.

Polymorphism of tmprss2 (rs12329760) but not ace2 (rs4240157), tmprss11a (rs353163) and cd147 (rs8259) is associated with the severity of COVID-19 in the Ukrainian population.

BACKGROUND AND AIM: Angiotensin-converting enzyme 2 (ACE2), transmembrane serine 2 and serine 11A proteases (TMPRSS2, TMPRSS11A), and a cell surface cluster of differentiation 147 (CD147) might be a gene candidate that exerts the susceptibility to and mortality from coronavirus disease 19 (COVID-19). The aim of this study was to investigate the associations between ace2, tmprss2, tmprss11a, and cd147 polymorphic variants and the severity of COVID-19 in the Ukrainian population. METHODS: The study population consisted of the Ukrainian population with COVID-19: patients without oxygen therapy (n=62), with non-invasive (n=92) and invasive (n=35) oxygen therapy, as well as control subjects (n=92). Allelic polymorphisms of ace2 rs4240157, tmprss2 rs12329760, and tmprss11a rs353163 were determined by real-time PCR, and cd147 rs8259 polymorphism was detected by PCR with subsequent restrictase analysis. We compared investigated polymorphisms distribution with other populations by meta-analysis. RESULTS: Our study is the first to obtain data about the distribution of investigated gene polymorphisms in the Ukrainian population: tmprss2 rs12329760 - CC 60.9%, CT 35.9%, TT 3.2%; tmprss11a rs353163 - CC 46.7%, CT 40.2%, TT 13.1%; ace2 rs4240157 - CC 7.6%, C 18.5%, CT 22.8%, TT 19.6%, T 31.5%; cd147 rs8259 - TT 60.9%, AT 32.6%, AA 6.5%. This distribution was similar to the Northern, Western and Southern European populations. There was a statistically significant difference in the frequency of tmprss2 polymorphic genotypes CC 57.1%, CT 28.6%, and TT 14.3% (P<0.05) in COVID-19 patients with invasive oxygen therapy in comparison with non-invasive oxygen therapy. This tmprss2 mutation occurs in the scavenger receptor cysteine-rich (SRCR) domain and might be important for protein-protein interaction in a calcium-dependent manner. CONCLUSIONS: Our study indicated the presence of an association between the tmprss2 rs12329760 polymorphism and the severity of COVID-19 in the Ukrainian population.

Polymorphism of tmprss2 (rs12329760) and severity of COVID-19 in the Ukrainian population: Response.

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Regulatory T cells suppress the motility of cytotoxic T cells in Friend retrovirus-infected mice.

Antiviral immunity often requires CD8+ cytotoxic T lymphocytes (CTLs) that actively migrate and search for virus-infected targets. Regulatory T cells (Tregs) have been shown to suppress CTL responses, but it is not known whether this is also mediated by effects on CTL motility. Here, we used intravital 2-photon microscopy in the Friend retrovirus (FV) mouse model to define the impact of Tregs on CTL motility throughout the course of acute infection. Virus-specific CTLs were very motile and had frequent short contacts with target cells at their peak cytotoxic activity. However, when Tregs were activated and expanded in late-acute FV infection, CTLs became significantly less motile and contacts with target cells were prolonged. This phenotype was associated with development of functional CTL exhaustion. Tregs had direct contacts with CTLs in vivo and, importantly, their experimental depletion restored CTL motility. Our findings identify an effect of Tregs on CTL motility as part of their mechanism of functional impairment in chronic viral infections. Future studies must address the underlying molecular mechanisms.

Patients with SARS-CoV-2-Induced Viral Sepsis Simultaneously Show Immune Activation, Impaired Immune Function and a Procoagulatory Disease State.

BACKGROUND: It is widely accepted that SARS-CoV-2 causes a dysregulation of immune and coagulation processes. In severely affected patients, viral sepsis may result in life endangering multiple organ dysfunction. Furthermore, most therapies for COVID-19 patients target either the immune system or coagulation processes. As the exact mechanism causing SARS-CoV-2-induced morbidity and mortality was unknown, we started an in-depth analysis of immunologic and coagulation processes. METHODS: 127 COVID-19 patients were treated at the University Hospital Essen, Germany, between May 2020 and February 2022. Patients were divided according to their maximum COVID-19 WHO ordinal severity score (WHO 0-10) into hospitalized patients with a non-severe course of disease (WHO 4-5, n = 52) and those with a severe course of disease (WHO 6-10, n = 75). Non-infected individuals served as healthy controls (WHO 0, n = 42). Blood was analyzed with respect to cell numbers, clotting factors, as well as pro- and anti-inflammatory mediators in plasma. As functional parameters, phagocytosis and inflammatory responses to LPS and antigen-specific stimulation were determined in monocytes, granulocytes, and T cells using flow cytometry. FINDINGS: In the present study, immune and coagulation systems were analyzed simultaneously. Interestingly, many severe COVID-19 patients showed an upregulation of pro-inflammatory mediators and at the same time clear signs of immunosuppression. Furthermore, severe COVID-19 patients not only exhibited a disturbed immune system, but in addition showed a pronounced pro-coagulation phenotype with impaired fibrinolysis. Therefore, our study adds another puzzle piece to the already complex picture of COVID-19 pathology implying that therapies in COVID-19 must be individualized. CONCLUSION: Despite years of research, COVID-19 has not been understood completely and still no therapies exist, fitting all requirements and phases of COVID-19 disease. This observation is highly reminiscent to sepsis. Research in sepsis has been going on for decades, while the disease is still not completely understood and therapies fitting all patients are lacking as well. In both septic and COVID-19 patients, immune activation can be accompanied by immune paralysis, complicating therapeutic intervention. Accordingly, therapies that lower immune activation may cause detrimental effects in patients, who are immune paralyzed by viral infections or sepsis. We therefore suggest individualizing therapies and to broaden the spectrum of immunological parameters analyzed before therapy. Only if the immune status of a patient is understood, can a therapeutic intervention be successful.

SARS-CoV-2-specific T cell responses wane profoundly in convalescent individuals 10 months after primary infection.

A key question in the coronavirus disease 2019 (COVID-19) pandemic is the duration of specific T cell responses against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) post primary infection, which is difficult to address due to the large-scale COVID-19 vaccination and re-exposure to the virus. Here, we conducted an analysis of the long-term SARS-CoV-2-specific T cell responses in a unique cohort of convalescent individuals (CIs) that were among the first to be infected worldwide and without any possible antigen re-exposure since then. The magnitude and breadth of SARS-CoV-2-specific T cell responses correlated inversely with the time that had elapsed from disease onset and the age of those CIs. The mean magnitude of SARS-CoV-2-specific CD4 and CD8 T cell responses decreased about 82% and 76%, respectively, over the time period of ten months after infection. Accordingly, the longitudinal analysis also demonstrated that SARS-CoV-2-specific T cell responses waned significantly in 75% of CIs during the follow-up. Collectively, we provide a comprehensive characterization of the long-term memory T cell response in CIs, suggesting that robust SARS-CoV-2-specific T cell immunity post primary infection may be less durable than previously expected.

Macrophage migration inhibitory factor receptor CD74 expression is associated with expansion and differentiation of effector T cells in COVID-19 patients.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused millions of COVID-19 cases and deaths worldwide. Severity of pulmonary pathologies and poor prognosis were reported to be associated with the activation non-virus-specific bystander T cells. In addition, high concentrations of the macrophage migration inhibitory factor (MIF) were found in serum of COVID-19 patients. We hypothesized that these two pathogenic factors might be related and analyzed the expression of receptors for MIF on T cells in COVID-19. T cells from PBMCs of hospitalized patients with mild and severe COVID-19 were characterized. A significantly higher proportion of CD4+ and CD8+ T cells from COVID-19 patients expressed CD74 on the cell surface compared to healthy controls. To induce intracellular signaling upon MIF binding, CD74 forms complexes with CD44, CXCR2, or CXCR4. The vast majority of CD74+ T cells expressed CD44, whereas expression of CXCR2 and CXCR4 was low in controls but increased upon SARS-CoV-2 infection. Hence, T cells in COVID-19 patients express receptors that render them responsive to MIF. A detailed analysis of CD74+ T cell populations revealed that most of them had a central memory phenotype early in infection, while cells with an effector and effector memory phenotype arose later during infection. Furthermore, CD74+ T cells produced more cytotoxic molecules and proliferation markers. Our data provide new insights into the MIF receptor and co-receptor repertoire of bystander T cells in COVID-19 and uncovers a novel and potentially druggable aspect of the immunological footprint of SARS-CoV-2.