Discussion
infection
INFECTION
This study describes cellular immune responses following bacterial challenge and evaluates which features of this response correlate with protection against colonization and/or seroconversion status. First, we evaluated the general immune cell kinetics after challenge. Next, we assessed differences in immune signature based on colonization, density, or seroconversion status. In the early days (D1, D3, and D4) after bacterial challenge, multiple cellular changes were observed in the blood, especially in the monocytic compartment, Tfh cells, and plasma cells. Kinetics of these subsets differed between colonized and colonization-protected participants, and surprisingly, the most prominent differences were found between low-density-colonized and colonization-protected participants. Interestingly, the D3 plasma cell expansion negatively correlated with CFU load on D7 and D9 after challenge. Later, on D11–D14 after challenge, a trend toward expansion of various plasma cell subsets was observed. Although this exploratory study consisted of only 15 wP-primed participants, which were clustered based on their colonization status (leading to low statistical power), we detected consistent kinetics related to clinical readouts (colonization and seroconversion status). These findings should be further confirmed in a larger study cohort.An important aspect of vaccine evaluation and improvement is in understanding the underlying immune response. To our knowledge, cellular immunity against In 2 previous vaccination studies, we explored the cellular responses to intramuscular aP booster vaccination in various cohorts using highly similar methods (aP-primed children, aP- or wP-primed adolescents, wP-primed young adults, and wP-primed older adults) (Of all evaluated innate immune cells, monocyte subsets hold promise for future analysis. The decrease in CD62LIn our previously published vaccination study, we did not observe the activation of cMos or the expansion of CD36The expansion of NK cells seemed to have an inverse relationship with colonization density. This expansion was not observed upon booster vaccination with aP, as was the case with intranasal study reports on NK cell kinetics (Interestingly, we found that low-density-colonized donors showed more prominent innate immune cell kinetics compared with high-density-colonized participants. A possible explanation might be the presence of some preexisting immunological memory, yet not as high as in participants protected against colonization. This preexisting memory might result in a more targeted innate immune response (influenced by preexisting memory cells or antibodies) and faster control of bacterial growth as compared with donors without any noteworthy preexisting immunological memory.Within the T cell compartment, we found an increase in Tfh cells on D3 after challenge in participants protected against colonization. This was not observed after aP booster vaccination, yet an expansion of activated Th1-like Tfh cells was observed from D4 after intranasal vaccination (with BPZE1) (In this study, we observed a trend toward expansion of several IgG and IgA plasma cell subsets on D3 (mostly in colonization-protected participants) and on D11–D14 after challenge. This differs from postvaccination kinetics, where we observed a prominent and homogeneous expansion and maturation of (primarily IgG1) plasma cells on D7 after vaccination. This difference may be explained by the manner of Ag encounter. During the booster vaccination, a high dose of Ag (plus adjuvant) is injected into the muscle, and thus all participants will be exposed to the Ag at the same time (D0), resulting in a synchronized induction of the immune response in vaccinated donors. In intranasal bacterial challenge, all participants are challenged with The observed IgG response is most likely the result of previous vaccine-induced immunity against In this study, all participants were inoculated with predefined doses of the well-characterized Not all participants who were colonized showed signs of seroconversion. Perhaps translocation of In this cellular immune monitoring study, we evaluated the overall immune cell kinetics in the blood. Immune cell populations in the blood have been well characterized in several studies and their phenotype and quantities can be relatively easily compared between studies. One of the limitations of this approach is that we may not capture part of the local responses occurring at the pathogen encounter site, i.e., within the (mucosal) tissues. We acknowledge that such cells would be a valuable source of information and their evaluation could be included in follow-up studies. However, due to the burdensome nature of this type of sample collection, it is unlikely that such an analysis would be included in any routine type of analysis. Another possibility to dissect cellular changes specific to colonization/pathogen encounter is to focus on changes in pathogen-specific cells of the adaptive immune system, i.e., B cells and T cells recognizing Here, we monitored the overall (non–Ag-specific) kinetics of immune cells. We previously showed that there is a fair correlation between Ag-specific ELISpot data and plasma cell flow cytometry data in postvaccination settings (One remaining question is whether the early response observed in participants protected against colonization is indeed protective, and to what extent this information can be used in future studies. To confirm whether a protective response is observed in participants, a rechallenge would indeed be required (e.g., an infection-reinfection cohort). As we know that natural infection gives the best protection (compared with wP and aP vaccination) (To the best of our knowledge, we are the first to investigate such in-depth cellular immune responses in a 