Highlights d Longitudinal analysis of multifaceted immune memory following mild COVID-19 d Antibodies capable of neutralizing virus persist for at least 3 months in most subjects d Virus-specific memory B and T cells display hallmarks of anti-viral immunity d MBCs increase in number and express antibodies capable of neutralizing SARS-CoV-2

The Toxoplasma gondii granule protein GRA15 activates the NF-κB pathway.

Stromal cells (SCs) establish the compartmentalization of lymphoid tissues critical to the immune response. However, the full diversity of lymph node (LN) SCs remains undefined. Using droplet-based single-cell RNA sequencing, we identified nine peripheral LN non-endothelial SC clusters. Included are the established subsets, Ccl19 T-zone reticular cells (TRCs), marginal reticular cells, follicular dendritic cells (FDCs), and perivascular cells. We also identified Ccl19 TRCs, likely including cholesterol-25-hydroxylase cells located at the T-zone perimeter, Cxcl9 TRCs in the T-zone and interfollicular region, CD34 SCs in the capsule and medullary vessel adventitia, indolethylamine N-methyltransferase SCs in the medullary cords, and Nr4a1 SCs in several niches. These data help define how transcriptionally distinct LN SCs support niche-restricted immune functions and provide evidence that many SCs are in an activated state.

Immunoglobulin A (IgA) induction primarily occurs in intestinal Peyer’s patches (PPs). However, the cellular interactions necessary for IgA class switching are poorly defined. Here we show that in mice, activated B cells use the chemokine receptor CCR6 to access the sup-epithelial dome (SED) of PPs. There, B cells undergo prolonged interactions with SED dendritic cells (DCs). PP IgA class switching requires innate lymphoid cells, which promote lymphotoxin-β receptor (LTβR)-dependent maintenance of DCs. PP DCs augment IgA production by integrin αvβ8-mediated activation of TGFβ. In mice where B cells cannot access the SED, IgA responses against oral antigen and gut commensals are impaired. These studies establish the PP SED as a niche supporting DC-B cell interactions needed for TGFβ activation and induction of mucosal IgA responses.