UCLA

The humoral immune response is mediated by antigen activated B cells that

have terminally differentiated into antibody secreting cells (ASCs). The ASC pool is

composed of short-lived plasma cells (SLPCs) and long-lived plasma cells (LLPCs) that

secrete antigen-specific antibodies to clear an infection and maintain long-term

protective antibody titers to prevent subsequent reinfections. SLPCs have generally

been viewed to be formed from T cell-independent immune responses and localized in

the spleen. LLPCs have been viewed to be formed from T cell-dependent immune

responses and localized in the bone marrow. However, regardless of the type of

stimulating antigen, ASCs of varying lifespans, both SLPCs and LLPCs are found in the

spleen and bone marrow. Currently, it remains unclear as to what factors and pathways

regulate PC longevity.

Our laboratory previously identified the CREB coactivator CRTC2 as a regulator

of ASC differentiation. DNA double strand breaks associated with class switch

recombination activates a signaling pathway in human germinal center (GC) B cells that

results in the phosphorylation and inactivation of CRTC2. Phosphorylated CRTC2 is relocalized

to the cytoplasm and CRTC2 target genes are down-regulated. Dysregulation

of CRTC2 activity through over-expression of a nucleus-localized and constitutively

active form of CRTC2 (CRTC2-AA) in human tonsillar B cells, prevented GC B cells

from exiting the GC reaction and inhibited ASC differentiation. However, it remained

unclear whether the function of CRTC2 in this in vitro differentiation system would be

recapitulated in vivo and whether CRTC2 played any other roles in an in vivo humoral

immune response.

To evaluate these questions, we generated a transgenic (TG) mouse model

which expresses CRTC2-AA at all stages of B cell development. Using these TG mice,

we demonstrate that Crtc2 repression in PCs is an intrinsic requirement for PC

metabolic fitness. Sustained CRTC2 activity shortened the survival of splenic and bone

marrow PCs which resulted in the reduction of long-lived PCs and antibody deficits in

response to immunizations and acute viral infection. We further demonstrated that TG

PCs adopt characteristics associated with SLPCs which include reduced antibody

secretion, glycolysis, oxidative metabolism, and spare respiratory capacity.

Mechanistically, Crtc2 repression is necessary for the fidelity of PC gene expression

and mRNA alternative-splicing programs, with both programs altered in TG PCs.

Combined, our results show that Crtc2 repression in PCs must occur to support PC

metabolism and extend PC survival and lifespan during a humoral immune response.

We hypothesize that the level of Crtc2 repression in differentiated ASCs determines

metabolic fitness and ultimately PC survival and longevity in the bone marrow.