GSE173673
Transcriptional regulation of cell fate decisions in early B cell activation.
The cell fate decisions between plasmablasts (PBs), germinal center B cells (GCBCs) and non-GC-derived early memory B cells (eMBCs) during early B cell activation determine the outcome of the immune responses to pathogens and vaccines. To characterize these poorly understood lineage choices, we dissected the early B cell responses to T-dependent antigen in mice by single-cell RNA-sequencing. Early after immunization, a homogenous population of activated precursors (APs) gave rise to a transient wave of PBs, followed a day later by the emergence of the first GCBCs, with the transcriptomes of both rapidly diverging from that of APs. The majority of APs started to withdraw from the cell cycle very early on and gave rise to eMBCs, a developmental transition that involved limited transcriptional changes. These events were controlled by antigen availability that rapidly declined soon after immunization, and provision of antigen excess interfered with the cell cycle exit of APs and induced a new wave of PBs. Fate mapping experiments revealed a prominent contribution of eMBCs to the overall MBC pool. Generation of the earliest GCBCs was tightly controlled by the transcriptional repressor Bhlhe40 in the absence of which this population was increased in numbers. Bhlhe40 also restrained in a cell intrinsic fashion the response of T follicular helper (TFH) cells, a specialized T helper cell subset required to mount the GC response. In B cells, Bhlhe40 executed its function in the first days after immunization by selectively restricting the generation of the earliest GCBCs but not of eMBCs or PBs. Conditional Bhlhe40 inactivation confirmed cell-autonomous functions of Bhlhe40 in both GCBCs and TFH cells, while the GC phenotype was further enhanced upon loss of Bhlhe40 in both cell types. This negative regulation of the GC reaction by Bhlhe40 was of crucial importance, as Bhlhe40-deficient mice with progressing age succumbed to a B cell lymphoma characterized by the accumulation of monoclonal GCBCs-like cells and polyclonal TFH cells in various tissues. Overall design: 23 bulk RNA-seq samples, 3 single cell RNA-seq, 10 bulk RNA-seq, 2 ChIP-seq