A proportion of patients surviving acute coronavirus disease 2019 (COVID-19) infection develop post-acute COVID syndrome (long COVID (LC)) lasting longer than 12 weeks. Here, we studied individuals with LC compared to age-and gender-matched recovered individuals without LC, unexposed donors and individuals infected with other coronaviruses. Patients with LC had highly activated innate immune cells, lacked naive T and B cells and showed elevated expression of type I IFN (IFN-β) and type III IFN (IFN-λ1) that remained persistently high at 8 months after infection. Using a log-linear classification model, we defined an optimal set of analytes that had the strongest association with LC among the 28 analytes measured. Combinations of the inflammatory mediators IFN-β, PTX3, IFN-γ, IFN-λ2/3 and IL-6 associated with LC with 78.5-81.6% accuracy. This work defines immunological parameters associated with LC and suggests future opportunities for prevention and treatment.Acute COVID-19, caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is characterized by a broad spectrum of clinical severity, from asymptomatic to fatal 1,2 . The immune response during acute illness contributes to both host defense and pathogenesis of severe ). Pronounced immune dysregulation with lymphopenia and increased expression of inflammatory mediators 3,4 have been described in the acute phase. Following acute COVID-19 infection, a proportion of patients develop physical and neuropsychiatric symptoms lasting longer than 12 weeks (known as Long COVID, chronic COVID syndrome or post-acute sequelae of )), henceforth denoted as LC. Although similar syndromes have been described following infection with SARS-CoV-1 (ref. 6 ) and Middle East respiratory syndrome-related coronavirus 7 , LC often develops after mild-to-moderate 9 ). Symptoms persisting 6 months were observed in 76% of hospitalized patients, with muscle weakness and fatigue being most frequently reported 10,11 . LC affects between 10% and 30% of community-managed COVID-19 cases 2 to 3 months after infection 12,13 and can persist >8 months after infection 14 . LC symptoms include severe relapsing fatigue, dyspnea, chest tightness, cough,

T cells use their T-cell receptors (TCRs) to discriminate between lower-affinity self and higher-affinity non-self pMHC antigens. The strength of this discrimination and the mechanisms that produce it remain controversial. Although a large number of mouse and human TCRs have now been characterised, they have not been used to precisely quantitate discrimination. Here, we systematically quantify the discrimination of TCRs using a discrimination power (α). Early influential studies on three mouse TCRs suggested that discrimination was nearly perfect (α ~ 9.0). In striking contrast, our analysis of published data on other mouse and human TCRs, and more recent data on the original mouse TCRs, produced significantly lower discrimination (α = 2.0). Although not perfect, the discriminatory power of TCR was greater than that of conventional receptors such as cytokine receptors, GPCRs, RTKs, and CARs (α ≤ 1). The revised discriminatory power of the TCR is readily explained by a kinetic proofreading mechanisms, and accounts for the ability of low affinity self-antigens to stimulate autoimmune and anti-tumour T cell responses.

T cells use their T-cell receptors (TCRs) to discriminate between lower-affinity self and higher-affinity non-self pMHC antigens. Although the discriminatory power of the TCR is widely believed to be near-perfect, technical difficulties have hampered efforts to precisely quantify it. Here, we describe a method for measuring very low TCR/pMHC affinities, and use it to measure the discriminatory power of the TCR, and the factors affecting it. We find that TCR discrimination, although enhanced compared with conventional cell-surface receptors, is imperfect: primary human T cells can respond to pMHC with affinities as low as KD ~1 mM. The kinetic proofreading mechanism fit our data, providing the first estimates of both the time delay (2.8 s) and number of biochemical steps (2.67) that are consistent with the extraordinary sensitivity of antigen recognition. Our findings explain why self pMHC frequently induce autoimmune diseases and anti-tumour responses, and suggest ways to modify TCR discrimination.