LAUSR

After the rapid spread of SARS-CoV-2 in Wuhan, China, at the beginning of 2020, about 1.5 million confirmed cases and over 80,000 deaths have been reported around 200 countries and territories all over the world and the number continues to increase. However, we still have limited knowledge of this new coronavirus, especially the interaction between SARS-CoV-2 and our immune system. In contrast with infected adults, the children have received more attention because of the lower infection rates and milder symptoms. Less than 1% of infected cases were aged 10 years or younger. Only 3.5% of SARS-COV-2 infected children had lymphocytopenia. It has been reported, coronaviruses, including SARS-CoV, MERS-CoV, and SARS-CoV-2, seem to cause fewer symptoms and less severe disease in children compared with adults. This phenomenon may be related to the differences in the immune responses against the infection of coronaviruses between children and adults. Here, we reported the characteristics of immune response after the SARS-CoV-2 attack in children and found that there is a protective humoral immunity in infected children, in which memory B cells and S-protein specific Abs against the SARS-CoV-2 have been detected. Our observation presents one possible explanation for the milder symptoms in children after exposure to SARS-CoV-2. We analyzed the T/B lymphocytes in PBMC and the production of antibodies in serum from confirmed cases in pediatrics. The respiratory samples obtained from six patients were all tested positive by RT-PCR for SARS-CoV-2. A cycle threshold value less than 35 was defined as a positive test. Mild cough and sore throat were common symptoms at disease onset among these six patients. None of the patients had diarrhea or dyspnea during illness. The body temperature of three patients was below 38 degrees at disease onset. Chest X-ray showed no pneumonia among three patients. The detailed clinical and epidemiological features of patient 1/2/3/4/5 have been reported previously. Patient 6 was admitted to Children’s Hospital of Fudan University on 6 Feb and started with mild symptoms as cough and sore throat without fever and other symptoms. Chest X-ray showed no pneumonia. She got infected by SARS-CoV-2 from the household but the interval between symptom onset and exposure to index case is unclear. All patients presented with mild respiratory infections and have been discharged. Informed consent was obtained from the parents or guardians of the patients infected and uninfected with SARS-CoV-2 for the publication of their clinical data. Ethical approval was provided by the Hospital Ethics Committee. Our results showed that pediatric patients had more active B-cell immune responses than uninfected children and obvious antigen-specific antibody production within 2–3 weeks after illness onset. The experiments demonstrated that the neutralizing antibody against the Spike protein of SARS-CoV-2 was detected. This result indicates that there is a protective humoral immunity in children after the SARS-CoV-2 attack. To have a signature picture of immune responses following the SARS-CoV-2 infection, the RNA prepared from peripheral blood mononuclear cells (PBMC) from a SARS-CoV-2 infected pediatric case and an uninfected control were subjected to RNA sequencing using for Illumina HiSeqTM 2000. An immune systemrelated GO category enrichment analysis was performed to gain insights into the biological roles of the differential expression genes. We found that B cell-related GO terms were significantly enriched and top of 14 main immune response-related GO categories (S-Fig. 1a). Groups of the differential expression genes were highly enriched in infected case, including mature Bcell differentiation involved in immune response (GO:0002322), positive regulation of humoral immune response (GO:0002922), B-cell activation involved in immune response (GO:0002312) and humoral immune response mediated by circulating immunoglobulin (GO:0002925) (Fig. 1a). The RNA-seq profiling indicates that there is an enhanced humoral immune response responding to SARS-CoV-2 infection in children. Flow cytometry analysis was performed to analyze T and B cells from four infected pediatric cases and five uninfected controls. The controls were patients hospitalized during the same period without SARS-CoV-2 infection. The T cell gating strategy was shown in S-Fig. 2a. The previous report showed that the white blood cell count (median: 7.35 × 10/L; normal range: 3.9–9.9 × 10/L) and lymphocyte count (median: 3.25 × 10/L; normal range: 1.2–4.0 × 10/L) were normal in these infected pediatric cases, which was different from the lymphocytopenia in infected adults. Similar to the unchanged lymphocyte count, the percentage of CD3+, CD4+, and CD8+ T cells between infected and uninfected cases were comparable (S-Fig. 2b). Expression of a chemokine receptor CCR7, in combination with the naive cell marker CD45RA, has been shown to discriminate naïve T cell (NT: CD45RA+CCR7+)