Distributed Adaptive Search in T Cells: Lessons From Ants

There are striking similarities between the strategies ant colonies use to forage for food and immune systems use to search for pathogens. Searchers (ants and cells) use the appropriate combination of random and directed motion, direct and indirect agent-agent interactions, and traversal of physical structures to solve search problems in a variety of environments. An effective immune response requires immune cells to search efficiently and effectively for diverse types of pathogens in different tissues and organs, just as different species of ants have evolved diverse search strategies to forage effectively for a variety of resources in a variety of habitats. Successful T cell search is required to initiate the adaptive immune response in lymph nodes and to eradicate pathogens at sites of infection in peripheral tissue. Ant search strategies suggest novel predictions about T cell search. In both systems, the distribution of targets in time and space determines the most effective search strategy. We hypothesize that the ability of searchers to sense and adapt to dynamic targets and environmental conditions enhances search effectiveness through adjustments to movement and communication patterns. We also suggest that random motion is a more important component of search strategies than is generally recognized. The behavior we observe in ants reveals general design principles and constraints that govern distributed adaptive search in a wide variety of complex systems, particularly the immune system.

Development of circulating CD4+ T-cell memory

The ability of circulating CD4⁺ T cells to retain memories of previous antigenic encounters is a cardinal feature of the adaptive immune system. Over the past two decades, since the first description of central and effector memory T cells, many studies have examined molecular mechanisms controlling CD8⁺ T-cell memory, with comparatively less research into CD4⁺ T-cell memory. Here, we review a number of seminal studies showing that circulating memory CD4⁺ T cells develop directly from effector cells; and in so doing, preserve features of their effector precursors. We examine mechanisms controlling the development and phenotypes of memory CD4⁺ T cells, and provide an updated model that accommodates both the central and effector memory paradigm and the diverse T helper cell classification system.

Stochastic Inheritance of Division and Death Times Determines the Size and Phenotype of CD8+ T Cell Families

After antigen stimulation cognate naïve CD8⁺ T cells undergo rapid proliferation and ultimately their progeny differentiates into short-lived effectors and longer-lived memory T cells. Although the expansion of individual cells is very heterogeneous, the kinetics are reproducible at the level of the total population of cognate cells. After the expansion phase, the population contracts, and if antigen is cleared, a population of memory T cells remains behind. Different markers like CD62L, CD27, and KLRG1 have been used to define several T cell subsets (or cell fates) developing from individual naïve CD8⁺ T cells during the expansion phase. Growing evidence from high-throughput experiments, like single cell RNA sequencing, epigenetic profiling, and lineage tracing, highlights the need to model this differentiation process at the level of single cells. We model CD8⁺ T cell proliferation and differentiation as a competitive process between the division and death probabilities of individual cells (like in the Cyton model). We use an extended form of the Cyton model in which daughter cells inherit the division and death times from their mother cell in a stochastic manner (using lognormal distributions). We show that this stochastic model reproduces the dynamics of CD8⁺ T cells both at the population and at the single cell level. Modeling the expression of the CD62L, CD27, and KLRG1 markers of each individual cell, we find agreement with the changing phenotypic distributions of these markers in single cell RNA sequencing data. Retrospectively re-defining conventional T-cell subsets by “gating” on these markers, we find agreement with published population data, without having to assume that these subsets have different properties, i.e., correspond to different fates.

Generation and Immune Regulation of CD4+CD25-Foxp3+ T Cells in Chronic Obstructive Pulmonary Disease

The imbalance of CD4⁺Foxp3⁺ T cell subsets is reportedly involved in abnormal inflammatory immune responses in patients with chronic obstructive pulmonary disease (COPD). However, the possible role of CD4⁺CD25^-Foxp3⁺ T cells in immune regulation in COPD remains to be investigated. In the current study, distribution and phenotypic characteristics of CD4⁺CD25^-Foxp3⁺ T cells from peripheral blood were determined by flow cytometry; the origin, immune function and ultimate fate of CD4⁺CD25^-Foxp3⁺ T cells were further explored in vitro. It was observed that circulating CD4⁺CD25^-Foxp3⁺ T cells were significantly increased in stable COPD patients (SCOPD) and resembled central memory or effector memory T cells. Compared with peripheral CD4⁺CD25⁺Foxp3⁺ T cells, peripheral CD4⁺CD25^-Foxp3⁺ T cells showed a lower expression of Foxp3, CTLA-4, HELIOS, and TIGIT, but a higher expression of CD127 and KI-67, suggesting that CD4⁺CD25^-Foxp3⁺ T cells lost the expression of Tregs-associated molecules following the reduction in CD25. Unexpectedly, our study found that transforming growth factor-β1 (TGFβ1) decreased CD25 expression and played a critical role in the generation of CD4⁺CD25^-Foxp3⁺ T cells from CD4⁺CD25⁺Foxp3⁺ T cells. Phenotypic analysis further revealed that both inducible and peripheral CD4⁺CD25^-Foxp3⁺ T cells exhibited the features of activated conventional T cells. Importantly, memory CD4⁺CD25^-Foxp3⁺ T cells facilitated the proliferation and differentiation of naïve CD4⁺ T cells into Th17 cells in the presence of IL-1β, IL-6, IL-23, and TGFβ1. Finally, a fraction of CD4⁺CD25^-Foxp3⁺ T cells, exhibiting instability and plasticity, were converted to Th17 cells when subjected to Th17 cell-polarizing condition. Taken together, we propose that TGFβ1 is responsible for the generation of CD4⁺CD25^-Foxp3⁺ T cells, and these cells functionally exert an auxiliary effect on Th17 cells generation and might perpetuate chronic inflammation in COPD.

Rethinking Communication in the Immune System: The Quorum Sensing Concept

Quorum sensing was first described as the communication process bacteria employ to coordinate changes in gene expression and therefore, their collective behavior in response to population density. Emerging new evidence suggests that quorum sensing can also contribute to the regulation of immune cell responses. Quorum sensing might be achieved by the ability of immune cells to perceive the density of their own populations or those of other cells in their environment; responses to alterations in cell density might then be coordinated via changes in gene expression and protein signaling. Quorum sensing mechanisms can regulate T and B cell as well as macrophage function. We posit that perturbations in quorum sensing may undermine the balance between diverse immune cell populations and predispose the host to immune abnormalities.

Paracrine costimulation of IFN-γ signaling by integrins modulates CD8 T cell differentiation

The cytokine IFN-γ is a critical regulator of immune system development and function. Almost all leukocytes express the receptor for IFN-γ, yet each cell type elicits a different response to this cytokine. Cell type-specific effects of IFN-γ make it difficult to predict the outcomes of the systemic IFN-γ blockade and limit its clinical application, despite many years of research. To better understand the cell-cell interactions and cofactors that specify IFN-γ functions, we focused on the function of IFN-γ on CD8 T cell differentiation. We demonstrated that during bacterial infection, IFN-γ is a dominant paracrine trigger that skews CD8 T cell differentiation toward memory. This skewing is preferentially driven by contact-dependent T cell-T cell (T-T) interactions and the localized IFN-γ secretion among activated CD8 T cells in a unique splenic microenvironment, and is less sensitive to concurrent IFN-γ production by other immune cell populations such as natural killer (NK) cells. Modulation of CD8 T cell differentiation by IFN-γ relies on a nonconventional IFN-γ outcome that occurs specifically within 24 hours following infection. This is driven by IFN-γ costimulation by integrins at T-T synapses, and leads to synergistic phosphorylation of the proximal STAT1 molecule and accelerated IL-2 receptor down-regulation. This study provides evidence of the importance of context-dependent cytokine signaling and gives another example of how cell clusters and the microenvironment drive unique biology.

Quorum sensing in the immune system

Quorum sensing is the regulation of gene expression programmes in response to changes in population density. It is probably best recognized as a mechanism through which bacterial communities can synchronize behaviours, such as biofilm formation and bioluminescence. This Comment article highlights the emerging evidence suggesting that quorum sensing also contributes to the regulation of immune cell responses.