Increased plasma cell numbers on D3 and D11–D14 after challenge.
SECONDARY
T cell help, given by Tfh cells in the secondary lymphoid organs, is required to activate B cells after Ag encounter (Upon challenge, fluctuations in the naive and memory B cell compartment were limited and not consistent between participants (Next, we grouped the participants based on colonization status. Here, we observed an expansion of plasma cells on D3, which was most prominent in colonization-protected participants, but found in 7 participants in total (Then, we divided participants based on colonization density. Aside from colonization-protected participants, the plasma cell expansion on D3 was observed in 1 low- and 1 high-density-colonized participant (As the expansion of plasma cells on D3 after challenge seemed more prominent in colonization-protected participants, we correlated the plasma cell expansion on D3 with the maximum CFU load and the CFU load on each day after challenge until the first day of azithromycin treatment. No correlation was found between the plasma cell expansion and the maximum CFU load. Instead, we found a negative correlation between plasma cell expansion on D3 and CFU load on D7 (Spearman’s Thus, within the B cell compartment, we observed most changes within the IgG and IgA plasma cell subsets, yet all were subtle and should therefore be confirmed in a larger study cohort. An early expansion of plasma cells on D3 after challenge was primarily observed in participants protected against colonization, whereas plasma cells on D11–D14 after challenge expanded irrespective of colonization status. A negative correlation was observed between CFU load and plasma cell expansion on D7 and D9 after challenge. Lastly, maturation of the IgG1 and IgA1 plasma cells was observed to some extent, especially toward an intermediate mature phenotype on D11–D14 after challenge.