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A naive B cell (green) captures antigen (red) from an FDC.
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On page 1485, Suzuki et al. catch naive B cells snatching antigens from follicular dendritic cells (FDCs) on film—or rather, on a hard-drive recording lymphoid follicles in real time. The interaction between these cells had been inferred for many years, but modern visualization tools had not yet been used to investigate the details of antigen capture.
A critical step in adaptive immunity occurs as B cells acquire antigens, proliferate, and differentiate within lymph nodes. FDCs aid this process by extending their long processes through lymphoid follicles, capturing antigens (together with complement) as they enter, and presenting them to naive B cells. Here, Suzuki and colleagues document B cell–FDC interactions from start to finish using two-photon microscopy.
The authors’ images confirm the reputation of FDCs as enduring antigen presenters. Naive B cells continued to pick up antigens from FDCs nine days after immunization, perhaps increasing the chance that rare or distant B cells will encounter their antigen.
FDCs and B cells spent about 6.5 minutes in contact. And during this period they captured large chunks of antigen, sometimes grabbing bits of the FDC as well. The authors suggest that high affinity B cells take a piece of the FDC in order to increase their antigenic bounty. Indeed, the cells bearing FDC tidbits glowed brightest with fluorescently labeled antigen. Whether the extra baggage alters B cell fate remains to be investigated.
Visualization required the use of high affinity antigen-B cell receptor combos, which are not necessarily the norm during primary immune responses. However, using flow cytometry, the authors confirm that FDCs hand off antigen to low affinity B cells in a similar manner.
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