Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Follicular Dendritic Cells

• Antigen Retention

• Germinal Center

Main Findings​

Follicular dendritic cells (FDC) are specialized cells from stromal origins, specialized in antigen capture and presentation to B cells in the germinal center. FDCs are crucial to select B cells that underwent affinity maturation, a hallmark of the humoral response. Antigens are retained by FDCs in their naïve form for extended period of time (1). However, the dynamics of antigen retention after multiple immunization remain unknown.

Following B cell activation and proliferation in the germinal center dark zone, B cells undergo selection in B cell follicle light zone. There, FDCs capture and retain antigen in their naïve state, protected from damage by storage in non-degradative endosomal vesicles that periodically cycle to the cell surface. Recent work using single-cell RNA sequencing has highlighted some diversity of FDCs in lymphoid organs (2). Yet, whether this heterogenicity translates into functional difference is not established.

In this preprint (not peer-reviewed), Martinez-Riano et al. evaluate the dynamics of antigen capture and retention by FDC in the germinal center following multiple immunizations. Authors used confocal microscopy to follow fluorescent antigen-immune complexes overtime in lymphoid tissues from mice. In the B cell follicle, FDCs form a network of cells, with increased density through the center, visible by increased complement receptor 2 (CR2) expression. Authors used CR2 expression to normalize the antigen retention by FDCs and found that 21 hours post-immunization the antigens (PE-immune complexes) localize throughout the FDC network. However, after 7 days and up to 56 days, the antigens are found accumulated in the center of the FDC network. The accumulation and retention of antigens was independent of adjuvant use, and was also observed using HIV-gp120 nanoparticles, a vaccine immunogen that triggers complement activation without IC formation. B cell receptor (BCR) activation was also needed, as BCR-transgenic mice were deprived of detectable FDC network, but the inhibition of CD40 signalling, while preventing the increased number of FDCs, did not impact the antigen accumulation and retention in the center of the FDC network. FDC, situated at the periphery of the FDC network and closer to the GC dark had lower expression of CR2 and Fc receptors as compared to those situated centrally. While antigen degradation capacities were similar for peripheral and central FDCs, the authors found that higher membrane levels of CR2 was responsible for the increased antigen retention on central FDCs as compared to peripheral FDCs.

Using droplet-based single cell RNA sequencing, the authors confirm previous results on FDC diversity in lymph nodes (2) and associated them with FDC spatial localization, either in periphery or centrally in the FDC network. While genes involved in antigen processing and cytoskeleton organization were the highest in the central FDCs accumulating the antigens, light zone peripheral FDCs showed highest expression of genes participating in the response to cytokines, MAP kinase and TNF signalling pathways, suggesting their higher responsiveness to extracellular signals.

Finally, multiple immunizations with various antigens partially saturated the FDCs and displaced previous antigens. However, mice mounted similar humoral responses whether the immunization was preceded by multiple FDC antigen loading.

Thus, while all FDCs capture the antigen following immunization, only the one situated centrally in the FDC network participate to the long-term antigen retention. This retention appears independent of GC-derived signals and is a consequence of higher levels of CR2 expression by central FDCs, preventing the fast dissociation of the antigens.

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Limitations​

• While the authors describe the impact of higher CR2 expression on antigen retention, how the antigen accumulate centrally is not clear. A role of B cells in antigen transfer from peripheral to central FDCs could be tested.

• As pointed by the authors, the mechanisms leading to the differences between peripheral and central FDC is not tested. Would the same FDC mature from the periphery to the center of the GC light zone?

• It would be of interest to evaluate whether antigen saturation of FDCs impairs secondary immunization in this model.

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Significance/Novelty

FDCs in the GC participate to the accumulation and retention of antigens for an extended period of time. This accumulation is associated with higher levels of CR2 expression, which prevent fast dissociation of the immune complexes from FDCs and increase their interaction half-life. Single-cell transcriptomics corroborates the functional heterogeneity of FDCs within the GC light zone. Using multiple immunizations, the authors showed that antigens partially saturated the FDCs and displaced previous antigens, without affecting the humoral response.

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References

1. Heesters, B. A., Myers, R. C. & Carroll, M. C. Follicular dendritic cells: dynamic antigen libraries. Nat. Rev. Immunol. 14, 495–504 (2014).

2. Pikor, N. B. et al. Remodeling of light and dark zone follicular dendritic cells governs germinal center responses. Nat. Immunol. 21, 649–659 (2020).

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Credit

Reviewed by Nicolas Ruffin as part of the cross-institutional journal club of the Immunology Institute of the Icahn School of Medicine, Mount Sinai, the Kennedy Institute of Rheumatology, University of Oxford, and Karolinska Institutet, Center for Infectious Medicine (CIM) & Center for Molecular Medicine (CIM).