An autoantibody signature predictive for multiple sclerosis

Although B cells are implicated in multiple sclerosis (MS) pathophysiology, a predictive or diagnostic autoantibody remains elusive. In this study, the Department of Defense Serum Repository (DoDSR), a cohort of over 10 million individuals, was used to generate whole-proteome autoantibody profiles of hundreds of patients with MS (PwMS) years before and subsequently after MS onset. This analysis defines a unique cluster in approximately 10% of PwMS who share an autoantibody signature against a common motif that has similarity with many human pathogens. These patients exhibit antibody reactivity years before developing MS symptoms and have higher levels of serum neurofilament light (sNfL) compared to other PwMS. Furthermore, this profile is preserved over time, providing molecular evidence for an immunologically active preclinical period years before clinical onset. This autoantibody reactivity was validated in samples from a separate incident MS cohort in both cerebrospinal fluid and serum, where it is highly specific for patients eventually diagnosed with MS. This signature is a starting point for further immunological characterization of this MS patient subset and may be clinically useful as an antigen-specific biomarker for high-risk patients with clinically or radiologically isolated neuroinflammatory syndromes.

Low protection from breakthrough SARS-CoV-2 infection and mild disease course in ocrelizumab-treated patients with multiple sclerosis after three mRNA vaccine doses

BACKGROUND

Our study investigated the rate of breakthrough SARS-CoV-2 infection and clinical outcomes in a cohort of multiple sclerosis (MS) patients who were treated with the anti-CD20 monoclonal antibody (Ab), ocrelizumab, before first, second and third BNT162b2 mRNA vaccinations. To correlate clinical outcomes with the humoral and cellular response.

METHODS

The study was a prospective non-randomised controlled multicentre trial observational study. Participants with a diagnosis of MS who were treated for at least 12 months with ocrelizumab prior to the first BNT162b2 mRNA vaccination were prospectively followed up from January 2021 to June 2022.

RESULTS

Out of 54 participants, 32 (59.3%) developed a positive SARS-CoV-2 PCR test in the study period. Mild infection was observed in all infected participants. After the third vaccination, the non-infected participants had higher mean Ab levels compared to the infected participants (54.3 binding antibody unit (BAU)/mL vs 26.5 BAU/mL, p=0.030). The difference in reactivity between spike-specific CD4+ and CD8+ T lymphocytes in the two groups was not significant.

CONCLUSION AND RELEVANCE

The study results demonstrate rates of 59% in breakthrough infections after the third SARS-CoV-2 mRNA vaccination in ocrelizumab-treated patients with MS, without resulting in critical disease courses. These findings suggest the need for continuous development of prophylactic treatments when proved important in the protection of severe breakthrough infection.

A common allele of HLA is associated with asymptomatic SARS-CoV-2 infection

Studies have demonstrated that at least 20% of individuals infected with SARS-CoV-2 remain asymptomatic1-4. Although most global efforts have focused on severe illness in COVID-19, examining asymptomatic infection provides a unique opportunity to consider early immunological features that promote rapid viral clearance. Here, postulating that variation in the human leukocyte antigen (HLA) loci may underly processes mediating asymptomatic infection, we enrolled 29,947 individuals, for whom high-resolution HLA genotyping data were available, in a smartphone-based study designed to track COVID-19 symptoms and outcomes. Our discovery cohort (n = 1,428) comprised unvaccinated individuals who reported a positive test result for SARS-CoV-2. We tested for association of five HLA loci with disease course and identified a strong association between HLA-B*15:01 and asymptomatic infection, observed in two independent cohorts. Suggesting that this genetic association is due to pre-existing T cell immunity, we show that T cells from pre-pandemic samples from individuals carrying HLA-B*15:01 were reactive to the immunodominant SARS-CoV-2 S-derived peptide NQKLIANQF. The majority of the reactive T cells displayed a memory phenotype, were highly polyfunctional and were cross-reactive to a peptide derived from seasonal coronaviruses. The crystal structure of HLA-B*15:01-peptide complexes demonstrates that the peptides NQKLIANQF and NQKLIANAF (from OC43-CoV and HKU1-CoV) share a similar ability to be stabilized and presented by HLA-B*15:01. Finally, we show that the structural similarity of the peptides underpins T cell cross-reactivity of high-affinity public T cell receptors, providing the molecular basis for HLA-B*15:01-mediated pre-existing immunity.

T cell deletional tolerance restricts AQP4 but not MOG CNS autoimmunity

Aquaporin-4 (AQP4)-specific Th17 cells are thought to have a central role in neuromyelitis optica (NMO) pathogenesis. When modeling NMO, only AQP4-reactive Th17 cells from AQP4-deficient (AQP4-/-), but not wild-type (WT) mice, caused CNS autoimmunity in recipient WT mice, indicating that a tightly regulated mechanism normally ensures tolerance to AQP4. Here, we found that pathogenic AQP4 T cell epitopes bind MHC II with exceptionally high affinity. Examination of T cell receptor (TCR) α/β usage revealed that AQP4-specific T cells from AQP4-/- mice employed a distinct TCR repertoire and exhibited clonal expansion. Selective thymic AQP4 deficiency did not fully restore AQP4-reactive T cells, demonstrating that thymic negative selection alone did not account for AQP4-specific tolerance in WT mice. Indeed, AQP4-specific Th17 cells caused paralysis in recipient WT or B cell-deficient mice, which was followed by complete recovery that was associated with apoptosis of donor T cells. However, donor AQP4-reactive T cells survived and caused persistent paralysis in recipient mice deficient in both T and B cells or mice lacking T cells only. Thus, AQP4 CNS autoimmunity was limited by T cell-dependent deletion of AQP4-reactive T cells. In contrast, myelin oligodendrocyte glycoprotein (MOG)-specific T cells survived and caused sustained disease in WT mice. These findings underscore the importance of peripheral T cell deletional tolerance to AQP4, which may be relevant to understanding the balance of AQP4-reactive T cells in health and in NMO. T cell tolerance to AQP4, expressed in multiple tissues, is distinct from tolerance to MOG, an autoantigen restricted in its expression.

Longitudinal adaptive immune responses following sequential SARS-CoV-2 vaccinations in MS patients on anti-CD20 therapies and sphingosine-1-phosphate receptor modulators

BACKGROUND

Adequate response to the SARS-CoV-2 vaccine represents an important treatment goal in caring for patients with multiple sclerosis (MS) during the ongoing COVID-19 pandemic. Previous data so far have demonstrated lower spike-specific IgG responses following two SARS-CoV-2 vaccinations in MS patients treated with sphingosine-1-phosphate (S1P) receptor modulators and anti-CD20 monoclonal antibodies (mAb) compared to other disease modifying therapies (DMTs). It is unknown whether subsequent vaccinations can augment antibody responses in these patients.

OBJECTIVES

The goal of this observational study was to determine the effects of a third SARS-CoV-2 vaccination on antibody and T cell responses in MS patients treated with anti-CD20 mAb or S1P receptor modulators.

METHODS

Vaccine responses in patients treated with anti-CD20 antibodies (ocrelizumab and ofatumumab) or S1P receptor modulators (fingolimod and siponimod) were evaluated before and after third SARS-CoV-2 vaccination as part of an ongoing longitudinal study. Total spike protein and spike receptor binding domain (RBD)-specific IgG responses were measured by Luminex bead-based assay. Spike-specific CD4+ and CD8+ T cell responses were measured by activation-induced marker expression.

RESULTS

MS patients and healthy controls were enrolled before and following SARS-CoV-2 vaccination. A total of 31 MS patients (n = 10 ofatumumab, n = 13 ocrelizumab, n = 8 S1P) and 10 healthy controls were evaluated through three SARS-CoV-2 vaccinations. Compared to healthy controls, total spike IgG was significantly lower in anti-CD20 mAb-treated patients and spike RBD IgG was significantly lower in anti-CD20 mAb and S1P-treated patients following a third vaccination. While seropositivity was 100% in healthy controls after a third vaccination, total spike IgG and spike RBD IgG seropositivity were lower in ofatumumab (60% and 60%, respectively), ocrelizumab (85% and 46%, respectively), and S1P-treated patients (100% and 75%, respectively). Longer treatment duration, including prior treatment history, appeared to negatively impact antibody responses. Spike-specific CD4+ and CD8+ T cell responses were well maintained across all groups following a third vaccination. Finally, immune responses were also compared in patients who were vaccinated prior to or following ofatumumab treatment. Antibody responses were significantly higher in those patients who received their primary SARS-CoV-2 vaccination prior to initiating ofatumumab treatment.

CONCLUSIONS

This study adds to the evolving understanding of SARS-CoV-2 vaccine responses in people with MS treated with disease-modifying therapies (DMTs) known to suppress humoral immunity. Our findings provide important information for optimizing vaccine immunity in at-risk MS patient populations.

A common allele of HLA mediates asymptomatic SARS-CoV-2 infection

Despite some inconsistent reporting of symptoms, studies have demonstrated that at least 20% of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) will remain asymptomatic. Although most global efforts have focused on understanding factors underlying severe illness in COVID-19 (coronavirus disease of 2019), the examination of asymptomatic infection provides a unique opportunity to consider early disease and immunologic features promoting rapid viral clearance. Owing to its critical role in the immune response, we postulated that variation in the human leukocyte antigen (HLA) loci may underly processes mediating asymptomatic infection. We enrolled 29,947 individuals registered in the National Marrow Donor Program for whom high-resolution HLA genotyping data were available in the UCSF Citizen Science smartphone-based study designed to track COVID-19 symptoms and outcomes. Our discovery cohort (n=1428) was comprised of unvaccinated, self-identified subjects who reported a positive test result for SARS-CoV-2. We tested for association of five HLA loci (HLA-A, -B, -C, -DRB1, -DQB1) with disease course and identified a strong association of HLA-B*15:01 with asymptomatic infection, and reproduced this association in two independent cohorts. Suggesting that this genetic association is due to pre-existing T-cell immunity, we show that T cells from pre-pandemic individuals carrying HLA-B*15:01 were reactive to the immunodominant SARS-CoV-2 S-derived peptide NQKLIANQF, and 100% of the reactive cells displayed memory phenotype. Finally, we characterize the protein structure of HLA-B*15:01-peptide complexes, demonstrating that the NQKLIANQF peptide from SARS-CoV-2, and the highly homologous NQKLIANAF from seasonal coronaviruses OC43-CoV and HKU1-CoV, share similar ability to be stabilized and presented by HLA-B*15:01, providing the molecular basis for T-cell cross-reactivity and HLA-B*15:01-mediated pre-existing immunity.

SARS-CoV-2 Variant Exposures Elicit Antibody Responses With Differential Cross-Neutralization of Established and Emerging Strains Including Delta and Omicron

The wide spectrum of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants with phenotypes impacting transmission and antibody sensitivity necessitates investigation of immune responses to different spike protein versions. Here, we compare neutralization of variants of concern, including B.1.617.2 (delta) and B.1.1.529 (omicron), in sera from individuals exposed to variant infection, vaccination, or both. We demonstrate that neutralizing antibody responses are strongest against variants sharing certain spike mutations with the immunizing exposure, and exposure to multiple spike variants increases breadth of variant cross-neutralization. These findings contribute to understanding relationships between exposures and antibody responses and may inform booster vaccination strategies.

Persistently reduced humoral and sustained cellular immune response from first to third SARS-CoV-2 mRNA vaccination in anti-CD20-treated multiple sclerosis patients

Objective

To examine humoral and cellular response in multiple sclerosis patients on anti-CD20 therapy after third BNT162b2 mRNA SARS-CoV-2 vaccination.

Methods

A prospective longitudinal study design from first throughout third vaccination in Danish and American MS centers. All participants were treated with ocrelizumab. Antibody (Ab) levels were assessed before and after third vaccination using SARS-CoV-2 IgG II Quant assay (Abbott Laboratories). B- and T-lymphocytes enumeration was done with BD Multitest™6-color TBNK reagent. Spike-specific T-cell responses were measured through PBMC stimulation with spike peptide pools (JPT Peptide Technologies).

Results

We found that 14.0%, 37.7%, and 33.3% were seropositive after first, second and third vaccination.

The median Ab-levels were 74.2 BAU/mL (range: 8.5–2427) after second vaccination, as well as 43.7 BAU/ml (range: 7.8–366.1) and 31.3 BAU/mL (range: 7.9–507.0) before and after third vaccination, respectively.

No difference was found in levels after second and third vaccination (p = 0.1475). Seropositivity dropped to 25.0% of participants before the third vaccination, a relative reduction of 33.3% (p = 0.0020). No difference was found between frequencies of spike reactive CD4⁺and CD8⁺ T-cells after second (0.65 ± 0.08% and 0.95 ± 0.20%, respectively) and third vaccination (0.99 ± 0.22% and 1.3 ± 0.34%, respectively).

Conclusion

In this longitudinal cohort we found no significant increased humoral or cellular response with administration of a third SARS-CoV-2 mRNA vaccination. These findings suggest the need for clinical strategies to include allowance of B cell reconstitution before repeat vaccination and/or provision of pre-exposure prophylactic monoclonal antibodies.

Multiple sclerosis therapies differentially impact SARS-CoV-2 vaccine-induced antibody and T cell immunity and function

BACKGROUND

Vaccine-elicited adaptive immunity is a prerequisite for control of SARS-CoV-2 infection. Multiple sclerosis (MS) disease-modifying therapies (DMTs) differentially target humoral and cellular immunity. A comprehensive comparison of the effects of MS DMTs on SARS-CoV-2 vaccine–specific immunity is needed, including quantitative and functional B and T cell responses.

METHODS

Spike-specific Ab and T cell responses were measured before and following SARS-CoV-2 vaccination in a cohort of 80 study participants, including healthy controls and patients with MS in 6 DMT groups: untreated and treated with glatiramer acetate (GA), dimethyl fumarate (DMF), natalizumab (NTZ), sphingosine-1-phosphate (S1P) receptor modulators, and anti-CD20 mAbs. Anti–spike-Ab responses were assessed by Luminex assay, VirScan, and pseudovirus neutralization. Spike-specific CD4⁺ and CD8⁺ T cell responses were characterized by activation-induced marker and cytokine expression and tetramer.

RESULTS

Anti-spike IgG levels were similar between healthy control participants and patients with untreated MS and those receiving GA, DMF, or NTZ but were reduced in anti-CD20 mAb– and S1P-treated patients. Anti-spike seropositivity in anti-CD20 mAb–treated patients was correlated with CD19⁺ B cell levels and inversely correlated with cumulative treatment duration. Spike epitope reactivity and pseudovirus neutralization were reduced in anti-CD20 mAb– and S1P-treated patients. Spike-specific CD4⁺ and CD8⁺ T cell reactivity remained robust across all groups, except in S1P-treated patients, in whom postvaccine CD4⁺ T cell responses were attenuated.

CONCLUSION

These findings from a large cohort of patients with MS exposed to a wide spectrum of MS immunotherapies have important implications for treatment-specific COVID-19 clinical guidelines.

FUNDING

NIH grants 1K08NS107619, K08NS096117, R01AI159260, R01NS092835, R01AI131624, and R21NS108159; NMSS grants TA-1903-33713 and RG1701-26628; Westridge Foundation; Chan Zuckerberg Biohub; Maisin Foundation.

Humoral immune response following SARS-CoV-2 mRNA vaccination concomitant to anti-CD20 therapy in multiple sclerosis

BACKGROUND

The immunogenicity of COVID-19 vaccine among patients receiving anti-CD20 monoclonal antibody (Ab) treatment has not been fully investigated. Detectable levels of SARS-CoV-2 immunoglobulin G (IgG) are believed to have a predictive value for immune protection against COVID-19 and is currently a surrogate indicator for vaccine efficacy.

OBJECTIVE

To determine IgG Abs in anti-CD20 treated patients with multiple sclerosis (MS).

METHOD

IgG Abs against SARS-CoV-2 spike receptor-binding domain were measured with the SARS-CoV-2 IgG II Quant assay (Abbott Laboratories) before and after vaccination (n = 60).

RESULTS

36.7% of patients mounted a positive SARS-CoV-2 spike Ab response after the second dose of vaccine. Five patients (8.3%) developed Abs >264 BAU/mL, another 12 patients (20%) developed intermediate Abs between 54 BAU/mL and 264 BAU/mL and five patients (8.3%) had low levels <54 BAU/mL. Of all seropositive patients, 63.6% converted from seronegative to seropositive after the 2nd vaccine.

CONCLUSION

Our study demonstrates decreased humoral response after BNT162b2 mRNA SARS-CoV-2 vaccine in MS patients receiving B-cell depleting therapy. Clinicians should advise patients treated with anti-CD20 to avoid exposure to SARS-CoV-2. Future studies should investigate the implications of a third booster vaccine in patients with low or absent Abs after vaccination.

Impact of multiple sclerosis disease-modifying therapies on SARS-CoV-2 vaccine-induced antibody and T cell immunity

Vaccine-elicited adaptive immunity is an essential prerequisite for effective prevention and control of coronavirus 19 (COVID-19). Treatment of multiple sclerosis (MS) involves a diverse array of disease-modifying therapies (DMTs) that target antibody and cell-mediated immunity, yet a comprehensive understanding of how MS DMTs impact SARS-CoV-2 vaccine responses is lacking. We completed a detailed analysis of SARS-CoV-2 vaccine-elicited spike antigen-specific IgG and T cell responses in a cohort of healthy controls and MS participants in six different treatment categories. Two specific DMT types, sphingosine-1-phosphate (S1P) receptor modulators and anti-CD20 monoclonal antibodies (mAb), resulted in significantly reduced spike-specific IgG responses. Longer duration of anti-CD20 mAb treatment prior to SARS-CoV-2 vaccination were associated with absent antibody responses. Except for reduced CD4+ T cell responses in S1P-treated patients, spike-specific CD4+ and CD8+ T cell reactivity remained robust across all MS treatment types. These findings have important implications for clinical practice guidelines and vaccination recommendations in MS patients and other immunosuppressed populations.

T cells take aim at a ubiquitous autoantigen in multiple sclerosis

CD4⁺ T cells from multiple sclerosis lesions target a ubiquitous self-antigen that is shared by gut commensal bacteria (Planas et al, this issue).

B-Cell Therapies in Multiple Sclerosis

B cells play a vital function in multiple sclerosis (MS) pathogenesis through an array of effector functions. All currently approved MS disease-modifying therapies alter the frequency, phenotype, or homing of B cells in one way or another. The importance of this mechanism of action has been reinforced with the successful development and clinical testing of B-cell-depleting monoclonal antibodies that target the CD20 surface antigen. Ocrelizumab, a humanized anti-CD20 monoclonal antibody, was approved by the Food and Drug Administration (FDA) in March 2017 after pivotal trials showed dramatic reductions in inflammatory disease activity in relapsing MS as well as lessening of disability progression in primary progressive MS. These and other clinical studies place B cells at the center of the inflammatory cascade in MS and provide a launching point for development of therapies that target selective pathogenic B-cell populations.

NFM Cross-Reactivity to MOG Does Not Expand a Critical Threshold Level of High-Affinity T Cells Necessary for Onset of Demyelinating Disease

Of interest to the etiology of demyelinating autoimmune disease is the potential to aberrantly activate CD4⁺ T cells due to cross-recognition of multiple self-epitopes such as has been suggested for myelin oligodendrocyte glycoprotein epitope 35-55 (MOG_35-55) and neurofilament medium protein epitope 15-35 (NFM_15-35). NFM_15-35 is immunogenic in C57BL/6 mice but fails to induce demyelinating disease by polyclonal T cells despite having the same TCR contact residues as MOG_35-55, a known encephalitogenic Ag. Despite reported cross-reactivity with MOG-specific T cells, the polyclonal response to NFM_15-35 did not expand threshold numbers of MOG_38-49 tetramer-positive T cells. Furthermore, NFM lacked functional synergy with MOG to promote experimental autoimmune encephalomyelitis because NFM-deficient synonymous with knockout mice developed an identical disease course to wild-type mice after challenge with MOG_35-55 Single-cell analysis of encephalitogenic T cells using the peptide:MHC monomer-based two-dimensional micropipette adhesion frequency assay confirmed that NFM was not a critical Ag driving demyelinating disease because NFM_18-30-specific T cells in the CNS were predominantly reactive to MOG_38-49 The absence of NFM contribution to disease allowed mapping of the amino acids required for encephalitogenicity and expansion of high-affinity, MOG-specific T cells that defined the polyclonal response. Alterations of N-terminal residues outside of the NFM_15-35 core nonamer promoted expansion of high-affinity, MOG_38-49 tetramer-positive T cells and promoted consistent experimental autoimmune encephalomyelitis induction, unlike mice challenged with NFM_15-35 Although NFM_15-35 is immunogenic and cross-reactive with MOG at the polyclonal level, it fails to expand a threshold level of encephalitogenic, high-affinity MOG-specific T cells.

A rare case of hyponatremia from a hypothalamic lesion in a patient with multiple sclerosis

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) can occur from a variety of neurologic and systemic processes; however, it has rarely been seen in multiple sclerosis (MS). We report a case of SIADH in a patient with MS and compare it with previously reported English-only cases. A 32-year-old woman experienced generalized fatigue followed by confusion and was found to have profound hyponatremia. Her work-up demonstrated SIADH secondary to a discrete enhancing hypothalamic lesion. Despite the seldom occurrence of SIADH in MS, hypothalamic lesions are more common than appreciated and should be considered in patients presenting with hyponatremia or endocrinopathy symptoms.

Low 2-dimensional CD4 T cell receptor affinity for myelin sets in motion delayed response kinetics

T cells recognizing self-peptides that mediate autoimmune disease and those that are responsible for efficacious immunity against pathogens may differ in affinity for antigen due to central and peripheral tolerance mechanisms. Here we utilize prototypical self-reactive (myelin) and viral-specific (LCMV) T cells from T cell receptor (TCR) transgenic mice (2D2 and SMARTA, respectively) to explore affinity differences. The T cells responsive to virus possessed >10,000 fold higher 2D affinity as compared to the self-reactive T cells. Despite their dramatically lower affinity for their cognate ligand, 2D2 T cells respond with complete, albeit delayed, activation (proliferation and cytokine production). SMARTA activation occurs rapidly, achieving peak phosphorylation of p38 (1 minute), Erk (30 minutes), and Jun (3 hours) as well as CD69 and CD25 upregulation (3 and 6 hours, respectively), with a corresponding early initiation of proliferation. 2D2 stimulation with MOG results in altered signaling--no phospho-Erk or phospho-p38 accumulation, significantly delayed activation kinetics of Jun (12 hours), and delayed but sustained SHP-1 activity--as well as delayed CD69 and CD25 expression (12-24 hours), and slow initiation of proliferation. This delay was not intrinsic to the 2D2 T cells, as a more potent antigen with >100-fold increased 2D affinity restored rapid response kinetics in line with those identified for the viral antigen. Taken together, these data demonstrate that time can offset low TCR affinity to attain full activation and suggest a mechanism by which low affinity T cells participate in autoimmune disease.

Self-reactive human CD4 T cell clones form unusual immunological synapses

Compared with influenza-specific T cells, self-reactive T cells from patients with multiple sclerosis or type 1 diabetes fail to slow down and do not form normal immunological synapses upon encounter with cognate self-peptide presented by MHC.

Recognition of self–peptide-MHC (pMHC) complexes by CD4 T cells plays an important role in the pathogenesis of many autoimmune diseases. We analyzed formation of immunological synapses (IS) in self-reactive T cell clones from patients with multiple sclerosis and type 1 diabetes. All self-reactive T cells contained a large number of phosphorylated T cell receptor (TCR) microclusters, indicative of active TCR signaling. However, they showed little or no visible pMHC accumulation or transport of TCR–pMHC complexes into a central supramolecular activation cluster (cSMAC). In contrast, influenza-specific T cells accumulated large quantities of pMHC complexes in microclusters and a cSMAC, even when presented with 100-fold lower pMHC densities. The self-reactive T cells also maintained a high degree of motility, again in sharp contrast to virus-specific T cells. 2D affinity measurements of three of these self-reactive T cell clones demonstrated a normal off-rate but a slow on-rate of TCR binding to pMHC. These unusual IS features may facilitate escape from negative selection by self-reactive T cells encountering very small amounts of self-antigen in the thymus. However, these same features may enable acquisition of effector functions by self-reactive T cells encountering large amounts of self-antigen in the target organ of the autoimmune disease.

High prevalence of low affinity peptide-MHC II tetramer-negative effectors during polyclonal CD4+ T cell responses

Two-dimensional analysis reveals that peptide–MHC class II tetramers underestimate the frequency of cytokine-producing antigen-specific CD4⁺ T cells in polyclonal responses.

T cell affinity for antigen initiates adaptive immunity. However, the contribution of low affinity cells to a response is unknown as it has not been possible to assess the entire affinity range of a polyclonal T cell repertoire. In this study, we used a highly sensitive two-dimensional binding assay to identify low affinity cells in polyclonal autoreactive and pathogen-reactive CD4⁺ T cell populations specific for myelin oligodendrocyte glycoprotein (MOG) and lymphocytic choriomeningitis virus (LCMV) antigens, respectively. Low affinity CD4⁺ T cells, below detection with peptide–major histocompatibility complex class II tetramers, were at least as frequent as high affinity responders and contributed significant effector cytokines in both primary antigen–specific responses. We further demonstrated that MOG- and LCMV-specific CD4⁺ T cells possessed similarly broad ranges in their affinities (>100-fold wide), only differing in the frequencies of low and high affinity cells. Thus, low as well as high affinity CD4⁺ T cells are critical effectors in autoimmune and pathogen-specific responses.

Loss of IFN-gamma enables the expansion of autoreactive CD4+ T cells to induce experimental autoimmune encephalomyelitis by a nonencephalitogenic myelin variant antigen

MHC variant peptides are analogues of immunogenic peptides involving alterations of the MHC-binding residues, thereby altering the affinity of the peptide for the MHC molecule. Recently, our laboratory demonstrated that immunization of WT B6 mice with 45D, a low-affinity MHC variant peptide of MOG(35-55), results in significantly attenuated experimental autoimmune encephalomyelitis (EAE), yet IFN-gamma production is comparable to myelin oligodendrocyte glycoprotein (MOG)(35-55)-immunized mice. In light of these findings, we asked whether IFN-gamma was required for the reduced encephalitogenicity of the weak ligand 45D in EAE. In this study, we report that immunization of mice deficient in IFN-gamma or its receptor with 45D exhibit significant EAE signs compared with 45D-immunized wild-type B6 mice. Moreover, 45D-immunized IFN-gamma(-/-) and IFN-gammaR(-/-) mice demonstrate MOG tetramer-positive CD4(+) T cells within the CNS and display substantial numbers of MOG-specific CD4(+) T cells in the periphery. In contrast, wild-type mice immunized with 45D exhibit reduced numbers of MOG-specific CD4(+) T cells in the periphery and lack MOG tetramer- positive CD4(+) T cells in the CNS. Importantly, the increased encephalitogenicity of 45D in mice lacking IFN-gamma or IFN-gammaR was not due to deviation toward an enhanced IL-17-secreting phenotype. These findings demonstrate that IFN-gamma significantly attenuates the encephalitogenicity of 45D and are the first to highlight the importance of IFN-gamma signaling in setting the threshold level of responsiveness of autoreactive CD4(+) T cells to weak ligands.

Mast cells are required for optimal autoreactive T cell responses in a murine model of multiple sclerosis

Once considered to be of sole importance in allergy and parasitic infections, the role of mast cells in other pathologic and protective immune responses is becoming increasingly evident. We previously demonstrated that mast cells contribute to the severity of EAE, the rodent model of multiple sclerosis. Here we show that one mode of mast cell action is through effects on the autoreactive T cell response. Early indices of both peripheral CD4 and CD8 T cell activation, including IFN-gamma production and increases in CD44 and CD11a expression, are attenuated in mast cell-deficient (W/Wv) mice after myelin oligodendrocyte glycoprotein(35-55) priming when compared to WT animals. Reduced infiltrates of activated T cells in the central nervous system are also observed. Importantly, selective repletion of the mast cell compartment restores most T cell responses in the lymph nodes and the central nervous system, correlating with reconstitution of severe disease. The adoptive transfer of WT-derived encephalitogenic T cells results in significantly less severe disease in W/Wv recipients, indicating that mast cells also exert potent effects after the initial T cell response is generated. Our data provide the first in vivo evidence that mast cells can significantly influence T cell responses and suggest that mast cells exacerbate disease during both the inductive and effector phases.