Persistence of vaccine-specific GC B cells after human influenza virus vaccination. (A) Schematic of study design. Eight healthy adults (aged 26–40) received the 2018 QIV intramuscularly. Blood, FNAs of the ipsilateral axillary lymph nodes (LN), and bone marrow (BM) were collected prior to vaccination (week 0) and at the indicated weeks after vaccination. (B) Kinetics of HA-binding PBs (CD20^lo HA⁺) in blood from seven participants. (C) 2018 QIV–specific IgG plasma antibody titers were measured via ELISA for seven participants. (D) Representative flow cytometry gating of total GC B cells (CD20⁺ CD38^int) and HA-binding GC B cells (CD20⁺ CD38^int HA⁺) in the LN from participant 05. Cells were pregated on CD4⁻ CD19⁺ IgD⁻ lymphocytes. (E) Kinetics of total GC B cells (open circles) and HA-binding GC B cells (closed circles) for all participants as defined by gating in D. Daggers indicate samples were excluded due to low cell recovery or blood contamination. (F) Representative ELISpot wells coated with 2018 QIV or anti-immunoglobulin (Ig) and developed in blue (IgG) and red (IgA) after plating BMPCs from participants 04, 05, and 11. (G) Frequencies of IgG and IgA 2018 QIV–specific BMPCs measured by ELISpot for seven participants. Participants with a detectable HA-binding GC are colored light blue. LoD, limit of detection. See also Fig. S1.