Induction of CD4 T cell memory by local cellular collectivity

Modeling memory differentiation in T cells The balance between effector and central memory T cells shifts toward the latter as the number of T cells participating in immune responses increases. Polonsky et al. determined the mechanisms by which T cell quorum sensing affects memory differentiation by using live-cell imaging to track cell proliferation and differentiation. They found that the rate of memory CD4+ T cell differentiation is determined by cell number. This rate substantially increases above a threshold number of locally interacting cells. Mathematical modeling suggests that the number of initially seeded cells and the number of cell divisions are not critical. Instead, the instantaneous number of interacting cells continuously modulates the differentiation rate. This is partly fueled by increased sensitivity to the cytokines interleukin-2 (IL-2) and IL-6, independent of any effects on cell proliferation. Science, this issue p. eaaj1853 Collective sensing by CD4+ T cells promotes differentiation of immunological memory T cells. INTRODUCTION Fate decisions play a central role in the operation of the vertebrate immune system. The establishment of efficient acquired immune responses depends on the differentiation of naïve T cells into various effector and memory cell types upon recognition of a cognate antigen, and on the appropriate balance between these populations. A number of studies have shown that the balance between effector and central memory T cells is shifted in favor of the latter when more T cells participate in the response. This observation has the hallmarks of quorum sensing, the ability of cells to respond to their population density. However, the mechanisms driving this behavior in T cells remain elusive. RATIONALE We observed increased differentiation of progenitor central memory T cells (pTCMs) at high cell densities both in vivo and in vitro. However, activated T cells rapidly form dense dynamic clusters, precluding the distinction between the effects of local interactions within cell clusters from global, long-range interactions through soluble factors. To overcome these difficulties, we used live-cell imaging to track the proliferation and differentiation of cells cultured in microwell arrays. This microculture system provides precise control and monitoring of the number of interacting T cells and their state after T cell activation. Continuously tracking differentiation and proliferation enabled us to investigate the mechanisms of cellular collectivity and its influence on memory differentiation. RESULTS We first validated that the pTCM cells formed early in our cultures show the markings of established central memory T cells using RNA sequencing and in vivo experiments. Then, with our microwell system, we showed that the rate of differentiation of pTCM cells is determined by the number of cells within individual microwells and sharply increases above a threshold number of locally interacting cells. Further analysis showed that cells follow a universal differentiation trajectory, whereby their differentiation rate is continuously modulated by the instantaneous number of interacting cells, rather than simply by the number of cells present initially within each microwell, or by the number of cell divisions. A combination of experimental manipulation and computational simulations showed that the observed collectivity involved increased sensitivity of clustered T cells to the cytokines interleukin-2 (IL-2) and IL-6, orthogonal to their effect on cell proliferation. CONCLUSION By systematically analyzing the role of intercellular interactions in a synthetic microenvironment, we showed that local T cell density could modulate the balance between central memory and effector cells independent of further potential influence by antigen-presenting cells or T cell receptor signaling strength. This cellular collectivity is a continuous process and is not determined by the number of cell divisions, but rather by the number of locally interacting cells at any given time. Local collectivity can influence the diversity and magnitude of immune memory, by modulating interactions between T cell clones during their priming in response to antigens. Understanding the rules of T cell social behavior will be important to learn how to manipulate the immune system for therapeutic or prophylactic goals. Collective local interactions enhance CD4+ memory T cell differentiation. The effects of intercellular interactions on T cell memory formation were studied in microwell arrays, each well holding a different number of locally interacting cells. Proliferation and differentiation were evaluated by using time-lapse movies. Differentiation into memory precursors sharply increased above a threshold number of interacting cells. This was modulated by increased sensitivity of the interacting cells to the cytokines IL-2 and IL-6. FIGURE COURTESY OF TAL BIGDARI/DIVISION OF RESEARCH SERVICES, WEIZMANN INSTITUTE OF SCIENCE Cell differentiation is directed by signals driving progenitors into specialized cell types. This process can involve collective decision-making, when differentiating cells determine their lineage choice by interacting with each other. We used live-cell imaging in microwell arrays to study collective processes affecting differentiation of naïve CD4+ T cells into memory precursors. We found that differentiation of precursor memory T cells sharply increases above a threshold number of locally interacting cells. These homotypic interactions involve the cytokines interleukin-2 (IL-2) and IL-6, which affect memory differentiation orthogonal to their effect on proliferation and survival. Mathematical modeling suggests that the differentiation rate is continuously modulated by the instantaneous number of locally interacting cells. This cellular collectivity can prioritize allocation of immune memory to stronger responses.