Humoral immunity after hematopoietic stem cell transplantation: evaluation by B-cell receptor repertoire analysis.

Immunity acquired before hematopoietic stem cell transplantation (HSCT) may decrease or disappear following HSCT, and it is unclear whether the first vaccination after HSCT in patients with an antigen exposure history before HSCT elicits a primary or secondary immune response. The Quantification of Antigen-Specific Antibody Sequence (QASAS) method enables real-time assessment of responses to SARS-CoV-2 antigen exposure through B-cell receptor (BCR) repertoire analysis. Using this method, we evaluated the disappearance of immunological memory after HSCT. First, in individuals without hematologic disorders, primary SARS-CoV-2 antigen exposure elicited no immune response at 7 days post-exposure but demonstrated activation between 14 to 21 days. In contrast, repeated exposure elicited early responses (secondary immune responses) at 7 days post-exposure. We then enrolled HSCT patients with pre-HSCT SARS-CoV-2 antigen exposure history and collected samples before and after vaccination. Despite prior exposure history, patients receiving their first vaccination after HSCT showed no response around 7 days post-exposure but responded at 14 days. In conclusion, even with pre-HSCT antigen exposure, the first vaccination after HSCT induced a primary immune response. This demonstrates that first vaccination after HSCT should be considered to induce a primary immune response, regardless of previous infection or vaccination history.