The germinal center (GC) reaction generates long-lived humoral immunity by producing memory B cells (Bmem) and long-lived plasma cells (LLPCs). The temporal regulation of these distinct B cell fates by T follicular helper (TFH) cells remains incompletely understood. This dissertation defines a temporal framework governing GC TFH specialization and subsequent GC B cell fate decisions. Using an inducible BCL6 knockout model for temporally controlled TFH depletion, combined with S1PR2-based fate and time-stamping, along with GC targeted CD40L blockade, we demonstrate that GC TFH cells present distinct functional states over the course of the GC reaction. Transcriptomic profiling of GC TFH cells at early (day 11), peak (day 19), and late (day 26) phases post-immunization revealed temporal shifts in key signaling molecules, including sequential deployment of IL-21, while maintaining elevated levels of IL-4, and CD40L, and a significant increase in genes involved in Type I Interferon Response during later stages. Critically, these TFH states map to distinct GC outputs: early GC TFH support is sufficient for GC expansion and Bmem formation, whereas sustained or durable TFH through peak and later GC stages were required to licenses the generation of LLPCs. By integrating genetic models, cell-surface markers, and transcriptomics, we establish that temporal shifts in the GC TFH population dictate specialized B cell fate decisions over the GC timeline.

Previous data from our laboratory demonstrated that abruptly disrupting TFH retention cues (integrins) skewed the immune response to generate Bmem cells at the expense of LLPCs. Other groups have suggested a temporal connection between the duration of the GC response and its outcomes, noting that early GC responses predominantly generate Bmem cells, while LLPC generation increases significantly over time. However, the exact cellular mechanisms driving this temporal shift remained unclear.

This work was driven by the central hypothesis that GC TFH cells temporally control these divergent B cell outcomes. Specifically, we hypothesized that the GC TFH population undergoes progressive transcriptional and functional transitions over the course of the immune response, and that these distinct temporal states are required to guide GC B cells into the long-lived pool. The quality and persistence of TFH help, not simply its magnitude, determines whether immunity proves transient or becomes life-long.

To explore this, chapter 1 is a comprehensive review of the major models for GC output regulation proposed prior to this work, detailing how TFH cells shape the quality, persistence, and magnitude of help required for antibody durability. Chapter 2 details the development and validation of the experimental tools and methodologies used to interrogate these temporal dynamics in vivo. Chapter 3 represents an original data manuscript demonstrating that the bifurcation of GC B cell fates is governed by a temporal and functional framework provided by GC TFH cells. This work reveals temporal changes in the GC TFH transcriptome during the GC reaction, that was not associated with key cytokines implicated in Bmem versus Plasma Cell fates such as IL-4, IFNg and IL-21, but was instead marked by a transition to an elevated ISG signature late in the GC reaction. To determine how these changes in transcription may regulate GC output, we deleted TFH at distinct time in the GC reaction. We found that GC TFH cells at early stages of the GC (Day 11) provide help that drives outputs toward the Bmem compartment but is not sufficient to support LLPCs. Sustained presence of TFH early and through the peak (Day 19) and late stage of the GC reaction (Day 26) was essential for LLPC development. Thus, there is a critical TFH “licensing window” required to latch high-affinity clones into the LLPC fate. This has important implications for the design of new vaccines, were sustaining TFH numbers may be particularly advantageous in the support of durable, long-lived protection.