Background
SARS-CoV-2 specific antibody-secreting plasma cells (PC) mediating specific humoral immunity have been identified in the human bone marrow (BM) after COVID-19 or vaccination against SARS-CoV-2. However, it remained unclear whether or not they acquire phenotypes of human memory plasma cells.
Methods
SARS-CoV-2-specific human bone marrow plasma cells (BMPC) were characterised by tetramer-based, antigen-specific flow cytometry and FluoroSpot assay. Findings: SARS-CoV-2 spike-S1-specific PC were detectable in all tested BM samples of previously vaccinated individuals, representing 0.22% of total BMPC. The majority of SARS-CoV-2-specific BMPC expressed IgG and their specificity for the spike S1 protein indicated emergence from a systemic vaccination response. Of note, one-fifth of SARS-CoV-2-specific BMPC showed the phenotype of memory plasma cells, i.e., downregulated CD19 and present or absent CD45 expression. Interpretation: Our data indicate the establishment of phenotypically diverse SARS-CoV-2-specific PC in the human BM after basic mRNA immunization, including the formation of memory phenotypes. These