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ImportanceThe immune response in children with SARS-CoV-2 infection is not well understood.ObjectiveTo compare seroconversion in children and adults with non-hospitalized (mild) SARS-CoV-2 infection and to understand the factors that influence this.DesignParticipants were part of a household cohort study of SARS-CoV-2 infection. Weekly nasopharyngeal/throat swabs and blood samples were collected during the acute and convalescent period following PCR diagnosis for analysis.SettingParticipants were recruited at the Royal Children’s Hospital, Melbourne, Australia between May and October 2020.ParticipantsThose who had a SARS-CoV-2 PCR-positive nasal/throat swab.Main outcomes and measuresSARS-CoV-2 antibody and cellular responses in children and adults. Seroconversion was defined by seropositivity in all three serological assays.ResultsAmong 108 SARS-CoV-2 PCR-positive participants, 57 were children (median age: 4, IQR 2-10) and 51 were adults (median age: 37, IQR 34-45). Using three established serological assays, a lower proportion of children seroconverted compared with adults [20/54 (37.0%) vs 32/42 (76.2%); (pConclusion and RelevanceIn this non-hospitalized cohort with mild COVID-19, children were less likely to seroconvert than adults despite similar viral loads. This has implications for future protection following COVID-19 infection in children and for interpretation of serosurveys that involve children. Further research to understand why children are less likely to seroconvert and develop symptoms following SARS-CoV-2 infection, and comparison with vaccine responses may be of clinical and scientific importance.Key pointsQuestionWhat proportion of children with non-hospitalized (mild) SARS-CoV-2 infection seroconvert compared to adults?FindingsIn this cohort study conducted in 2020, we found the proportion of children who seroconverted to SARS-CoV-2 was half that in adults despite similar viral load.MeaningSerology is a less reliable marker of prior SARS-CoV-2 infection in children. SARS-CoV-2-infected children who do not seroconvert may be susceptible to reinfection. Our findings support strategies to protect children against COVID-19 including vaccination.

CD1c presents lipid-based antigens to CD1c-restricted T cells which are thought to be a major component of the human T cell pool. The study of CD1c-restricted T cells, however, is hampered by the presence of an abundantly expressed CD1c-binding partner on blood cells distinct to the T cell receptor (TCR), confounding analysis of TCR-mediated CD1c tetramer staining. Here, we identify the CD36 family (CD36, CD36-L1 and CD36-L2) as novel ligands for CD1c, CD1b and CD1d proteins, and show that CD36 is the receptor responsible for non-TCR-mediated CD1c tetramer staining of blood cells. Moreover, CD36-blockade enables tetramer-based identification of CD1c-restricted T cells and clarifies identification of CD1b- and CD1d-restricted T cells. We use this technique to characterise CD1c-restricted T cells ex vivo and show diverse phenotypic features, TCR repertoire and antigen-specific subsets. Accordingly, this work will enable further studies into the biology of CD1 and human CD1-restricted T cells.One Sentence SummaryCD1 molecules bind CD36 family members and blockade of this interaction facilitates the study of CD1-restricted T cells.

Natural Killer T (NKT) cells detect lipids presented by CD1d. Most studies focus on type I NKT cells that express semi-invariant αβ T cell receptors (TCR) and recognise α-galactosylceramides. However, CD1d also presents structurally distinct lipids to NKT cells expressing diverse TCRs (type II NKT cells) but our knowledge of the antigens for type II NKT cells is limited. An early study identified an NKT cell agonist, phenyl pentamethyldihydrobenzofuransulfonate (PPBF), which is notable for its similarity to common sulfa-drugs, but its mechanism of NKT-cell activation remained unknown. Here we demonstrate that a range of pentamethylbenzofuransulfonate (PBFs), including PPBF, activate polyclonal type II NKT cells from human donors. Whereas these sulfa drug-like molecules might have acted pharmacologically on cells, here we demonstrate direct contact between TCRs and PBF-treated CD1d complexes. Further, PBF-treated CD1d-tetramers identified type II NKT cell populations cells expressing αβ and γδTCRs, including those with variable and joining region gene usage (TRAV12-1–TRAJ6) that was conserved across donors. By trapping a CD1d-type II NKT TCR complex for direct mass spectrometric analysis, we detected molecules that allow binding of CD1d to TCRs, finding that both PBF and short-chain sphingomyelin lipids are present in these complexes. Furthermore, the combination of PPBF and short-chain sphingomyelin enhances CD1d tetramer staining of PPBF-reactive T cell lines over either molecule alone. This study demonstrates that non-lipidic small molecules, that resemble sulfa-drugs implicated in systemic hypersensitivity and drug allergy reactions, activate a polyclonal population of type II NKT cells in a CD1d-restricted manner.Significance StatementWhereas T cells are known to recognize peptide, vitamin B metabolite or lipid antigens, we identify several non-lipidic small molecules, pentamethylbenzofuransulfonates (PBFs), that activate a population of CD1d-restricted NKT cells. This represents a breakthrough in the field of NKT cell biology. This study also reveals a previously unknown population of PBF-reactive NKT cells in healthy individuals with stereotyped receptors that paves the way for future studies of the role of these cells in immunity, including sulfa-drug hypersensitivity.