Quantification of plasma cell dynamics using mathematical modelling

Intrinsically determined turnover underlies broad heterogeneity in plasma-cell lifespan

Antibodies produced by antibody-secreting plasma cells (ASCs) underlie multiple forms of long-lasting immunity. Here we examined the mechanisms regulating ASC turnover and persistence using a genetic reporter to time-stamp ASCs. This approach revealed ASC lifespans as heterogeneous and falling on a continuum, with only a small fraction surviving for >60 days. ASC longevity past 60 days was independent of isotype but correlated with a phenotype that developed progressively and ultimately associated with an underlying "long-lived" ASC (LL ASC)-enriched transcriptional program. While some of the differences between LL ASCs and other ASCs appeared to be acquired with age, other features were shared with some younger ASCs, such as high CD138 and CD93. Turnover was unaffected by altered ASC production, arguing against competition for niches as a major driver of turnover. Thus, ASC turnover is set by intrinsic lifespan limits, with steady-state population dynamics governed by niche vacancy rather than displacement.

Making sense of plasma cell heterogeneity

Plasma cells (PCs) are essential for the quality and longevity of protective immunity. The canonical humoral response to vaccination involves induction of germinal centers in lymph nodes followed by maintenance by bone marrow-resident PCs, although there are many variations of this theme. Recent studies have highlighted the importance of PCs in nonlymphoid organs, including the gut, central nervous system, and skin. These sites harbor PCs with distinct isotypes and possible immunoglobulin-independent functions. Indeed, bone marrow now appears unique in housing PCs derived from multiple other organs. The mechanisms through which the bone marrow maintains PC survival long-term and the impact of their diverse origins on this process remain very active areas of research.

Long-lived plasma cells accumulate in the bone marrow at a constant rate from early in an immune response

Vaccines work largely by generating long-lived plasma cells (LLPCs), but knowledge of how such cells are recruited is sparse. Although it is clear that LLPCs preferentially originate in germinal centers (GCs) and relocate to survival niches in bone marrow where they can persist for decades, the issues of the timing of LLPC recruitment and the basis of their retention remain uncertain. Here, using a genetic timestamping system in mice, we show that persistent PCs accrue in bone marrow at an approximately constant rate of one cell per hour over a period spanning several weeks after a single immunization with a model antigen. Affinity-based selection was evident in persisting PCs, reflecting a relative and dynamic rather than absolute affinity threshold as evidenced by the changing pattern of V H gene somatic mutations conveying increased affinity for antigen. We conclude that the life span of persistent, antigen-specific PCs is in part intrinsic, preprogrammed, and varied and that their final number is related to the duration of the response in a predictable way. This implies that modulating vaccines to extend the duration of the GC reaction will enhance antibody-mediated protective immunity.

Data-Driven Mathematical Model of Apoptosis Regulation in Memory Plasma Cells

Memory plasma cells constitutively produce copious amounts of antibodies, imposing a critical risk factor for autoimmune disease. We previously found that plasma cell survival requires secreted factors such as APRIL and direct contact to stromal cells, which act in concert to activate NF-κB- and PI3K-dependent signaling pathways to prevent cell death. However, the regulatory properties of the underlying biochemical network are confounded by the complexity of potential interaction and cross-regulation pathways. Here, based on flow-cytometric quantification of key signaling proteins in the presence or absence of the survival signals APRIL and contact to the stromal cell line ST2, we generated a quantitative model of plasma cell survival. Our model emphasizes the non-redundant nature of the two plasma cell survival signals APRIL and stromal cell contact, and highlights a requirement for differential regulation of individual caspases. The modeling approach allowed us to unify distinct data sets and derive a consistent picture of the intertwined signaling and apoptosis pathways regulating plasma cell survival.

The Maintenance of Memory Plasma Cells

It is now well accepted that plasma cells can become long-lived (memory) plasma cells and secrete antibodies for months, years or a lifetime. However, the mechanisms involved in this process of humoral memory, which is crucial for both protective immunity and autoimmunity, still are not fully understood. This article will address a number of open questions. For example: Is longevity of plasma cells due to their intrinsic competence, extrinsic factors, or a combination of both? Which internal signals are involved in this process? What factors provide external support? What survival factors play a part in inflammation and autoreactive disease? Internal and external factors that contribute to the maintenance of memory long-lived plasma cells will be discussed. The aim is to provide useful additional information about the maintenance of protective and autoreactive memory plasma cells that will help researchers design effective vaccines for the induction of life-long protection against infectious diseases and to efficiently target pathogenic memory plasma cells.

Stem cell homeostasis by integral feedback through the niche

Hematopoiesis is a paradigm for tissue development and renewal from stem cells. Experiments show that the maintenance of hematopoietic stem cells (HSCs) relies on signals from niche cells. However, it is not known how the size of the HSC compartment is set. Competition by HSCs for niche access has been suggested, yet niche cells in the bone marrow outnumber HSCs. Here we propose a cooperative model of HSC homeostasis in which stem and niche cells mutually interact such that niche cells function as negative feedback regulators of HSC proliferation. This model explains puzzling experimental findings, including homeostatic recovery of the HSC compartment after irradiation versus apparent lack of recovery after HSC ablation. We show that bidirectional niche-stem cell regulation has properties of a proportional-integral feedback controller. Moreover, we predict that the outflux of differentiated cells from HSCs can be regulated by the affinity of HSCs for niche cells. Much effort has been devoted to elucidating niche cell signaling to stem cells; our theoretical insights indicate that studying the effect of stem cells on the niche may be equally important for understanding stem cell homeostasis.