Pediatric Coronavirus Disease 2019 (pCOVID-19) is rarely severe; however, a minority of children infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) might develop multisystem inflammatory syndrome in children (MIS-C), with substantial morbidity. In this longitudinal multi-institutional study, we applied multi-omics (analysis of soluble biomarkers, proteomics, single-cell gene expression and immune repertoire analysis) to profile children with COVID-19 (n = 110) and MIS-C (n = 76), along with pediatric healthy controls (pHCs; n = 76). pCOVID-19 was characterized by robust type I interferon (IFN) responses, whereas prominent type II IFN-dependent and NF-κB-dependent signatures, matrisome activation and increased levels of circulating spike protein were detected in MIS-C, with no correlation with SARS-CoV-2 PCR status around the time of admission. Transient expansion of TRBV11-2 T cell clonotypes in MIS-C was associated with signatures of inflammation and T cell activation. The association of MIS-C with the combination of HLA A*02, B*35 and C*04 alleles suggests genetic susceptibility. MIS-C B cells showed higher mutation load than pCOVID-19 and pHC. These results identify distinct immunopathological signatures in pCOVID-19 and MIS-C that might help better define the pathophysiology of these disorders and guide therapy.c g

Acute viral infections can have durable functional impacts on the immune system long after recovery, but how they affect homeostatic immune states and responses to future perturbations remain poorly understood [1][2][3][4] . Here we use systems immunology approaches, including longitudinal multimodal single cell analysis (surface proteins, transcriptome, and V(D)J sequences), to comparatively assess baseline immune statuses and responses to influenza vaccination in 33 healthy individuals after recovery from mild, non-hospitalized COVID-19 (mean: 151 days after diagnosis) and 40 age-and sex-matched controls who never had COVID-19. At baseline and independent of time since COVID-19, recoverees had elevated T-cell activation signatures and lower expression of innate immune genes in monocytes.COVID-19-recovered males had coordinately higher innate, influenza-specific plasmablast, and antibody responses after vaccination compared to healthy male and COVID-19-recovered females, partly because male recoverees had monocytes with higher IL-15 responses early after vaccination coupled with elevated pre-vaccination frequencies of "virtual memory" like CD8+ T-cells poised to produce more IFNγ upon IL-15 stimulation. In addition, the expression of the repressed innate immune genes in monocytes increased by day 1 through day 28 post-vaccination in recoverees, thus moving towards the pre-vaccination baseline of healthy controls. In contrast, these genes decreased on day 1 and returned to the baseline by day 28 in controls. Our study reveals sex-dimorphic impacts of prior mild COVID-19 and suggests that viral infections in humans can establish new set-points impacting future immune responses in an antigen-agnostic manner.

Most studies of adaptive immunity to SARS-CoV-2 infection focus on peripheral blood, which may not fully reflect immune responses at the site of infection. Using samples from 110 children undergoing tonsillectomy and adenoidectomy during the COVID-19 pandemic, we identified 24 samples with evidence of previous SARS-CoV-2 infection, including neutralizing antibodies in serum and SARS-CoV-2-specific germinal center and memory B cells in the tonsils and adenoids. Single-cell B cell receptor (BCR) sequencing indicated virus-specific BCRs were class-switched and somatically hypermutated, with overlapping clones in the two tissues. Expanded T cell clonotypes were found in tonsils, adenoids and blood post-COVID-19, some with CDR3 sequences identical to previously reported SARS-CoV-2-reactive T cell receptors (TCRs). Pharyngeal tissues from COVID-19-convalescent children showed persistent expansion of germinal center and antiviral lymphocyte populations associated with interferon (IFN)-γ-type responses, particularly in the adenoids, and viral RNA in both tissues. Our results provide evidence for persistent tissue-specific immunity to SARS-CoV-2 in the upper respiratory tract of children after infection.