The coronavirus spike (S) glycoprotein attaches to host receptors and mediates viral fusion. Using a broad screening approach, we isolated from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immune donors seven monoclonal antibodies (mAbs) that bind to all human-infecting coronavirus S proteins. This class of mAbs recognize the fusion peptide and acquire affinity and breadth through somatic mutations. Despite targeting a conserved motif, only some mAbs show broad neutralizing activity in vitro against alpha- and beta-coronaviruses, including animal coronavirus WIV-1 and PDF-2180. Two selected mAbs also neutralize Omicron BA.1 and BA.2 authentic viruses and reduce viral burden and pathology in vivo. Structural and functional analyses show that the fusion peptide-specific mAbs bind with different modalities to a cryptic epitope, which is hidden in prefusion stabilized S, and becomes exposed upon binding of angiotensin-converting enzyme 2 (ACE2) or ACE2-mimicking mAbs.

Highlights d Longitudinal profiling of anti-H1N1 serum antibodies (Abs) reveals persisting Abs d The persistent Abs on average account for >70% of the serum responses over 5 years d Most persistent Abs bind and neutralize a highly divergent H5N1 viral strain d Cross-neutralizing anti-influenza Abs can persist in the circulation Authors

The importance of CD4+ T helper (Th) cells is well appreciated in view of their essential role in the elicitation of antibody and cytotoxic T cell responses. However, the mechanisms that determine the selection of immunodominant epitopes within complex protein antigens remain elusive. Here, we used ex vivo stimulation of memory T cells and screening of naive and memory T cell libraries, combined with T cell cloning and TCR sequencing, to dissect the human naive and memory CD4+ T cell repertoire against the influenza pandemic H1 hemagglutinin (H1-HA). We found that naive CD4+ T cells have a broad repertoire, being able to recognize naturally processed as well as cryptic peptides spanning the whole H1-HA sequence. In contrast, memory Th cells were primarily directed against just a few immunodominant peptides that were readily detected by mass spectrometry–based MHC-II peptidomics and predicted by structural accessibility analysis. Collectively, these findings reveal the presence of a broad repertoire of naive T cells specific for cryptic H1-HA peptides and demonstrate that antigen processing represents a major constraint determining immunodominance.

Highlights d Analysis of HV2-70-derived antibodies reveals basis for a multidonor ''LPAF-a'' class d LPAF-a.01 antibody inserts extended CDR H3 into influenza sialic acid-binding site d Germline-reverted LPAF-a class antibodies have nanomolar affinity for CA09 antigen d Increased frequency of LPAF-a class antibodies in humans after CA09 vaccination