*Purpose: We describe the clinical presentation & immune response to COVID-19 infection in pediatric SOT.

*Methods: Medical records of COVID-19 PCR+ or seropositive patients were reviewed for details of their disease course. Blood was obtained during PCR or seropositivity for immunophenotyping & PlexCOVID-19 test. PlexCOVID-19 measures frequencies of spike Ag reactive T cells that express CD154. A pre-established algorithm predicts likelihood of COVID-19 severity. Controls were peds SOT patients with negative COVID-19 status. RNA isolated from liver tissue for PCR using the Lyra SARS CoV-2 Assay. As induction of immunological memory is central to anti-pathogen adaptive immunity induced by infection, IHC of liver tissue for tissue resident memory cells (T_RM) (defined as CD69⁺ expressing CD4 or CD8 T cells) was performed.

*Results: 4 patients had COVID-19 & 5 patients were seropositive between March & Nov 2020 (Table 1). The 2 symptomatic PCR+ patients were hospitalized for 24-48-hours & the symptomatic seropositive patient had a prolonged PICU stay. Steroid was discontinued in 1 symptomatic PCR+ patient; & target CNI goal & steroid dose decreased in the symptomatic seropositive patient due to concurrent BK viremia. 8 patients remain well at home. Histology on 1 symptomatic PCR+ patient with elevated LFT’s revealed lymphocytic portal inflammation & SARS CoV-2 PCR on the liver was negative; histology on 1 asymptomatic seropositive patient was normal. Infrequent T_RM were seen on liver biopsies but were increased in PCR+ & seropositive biopsies vs. biopsies from same patients that pre-date a COVID-19+ status. CD4⁺ T cells in PCR+ & seropositive patients had a phenotype consistent with activation, including expression of HLA-DR (p = 0.008); Further, BST2 was constitutively expressed on a subset of CD4+ T cells in PCR+ patients reflecting a history of IFN-alpha induced signals (p=0.01). The frequencies of spike Ag-reactive CD3 (0.95 ± 0.35 vs 2.73 ± 0.35, p= 0.037) and CD8 cells (1.10 ± 0.70 vs 5.03 ± 0.80, p= 0.034) were lower in symptomatic PCR+ patients compared with asymptomatic seropositive subjects.

*Conclusions: A small number of our SOT patients had mild or asymptomatic COVID-19 infection, with notable activation of CD4⁺ T cells, & constitutive expression of BST2 reflecting IFN-alpha induced signals. Spike-antigen-reactive T-cells was lower during symptomatic vs. asymptomatic infection.

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