Fighting Together?: T-helper Cells and Collective Decision-Making in the Mammalian Immune System

Abstract

T-helper cells are an important component of mammalian immunity, and their effector functions are traditionally characterized into stable subsets that target different pathogens (Abbas et al., 2016; Latif, 2018; Schrom & Graham, 2017). Despite the traditional understanding that T-helper cell subset differentiation is specific and stable, there is increasing evidence that differentiated Thelper cells are plastic and variable (DuPage & Bluestone, 2016; Latif, 2018; Nakayamada, Takahashi, Kanno, & O’Shea, 2012; Schrom & Graham, 2017), conflicting with the classical view of a cohesive T-helper cell-mediated immune response. Still, Schrom & Graham 2017 suggest a resolution to this conflict by comparing T-helper cells to eusocial insects and suggesting that Thelper cells behave as a swarm, where variable individuals collaborate to mount a unified immune response (Schrom & Graham, 2017). For this framework to be applicable, however, T-helper cells must perform collective decision-making, and T-helper behavior (i.e., effector choice) should be influenced by local social signals sent by other T-helper cells. Thus, this thesis investigated whether cytokines, signaling molecules produced by T-helper cells, may act as local social signals that influence T-helper cell differentiation. To study the influence of cytokines on T-helper cell effector choice, T-helper cells with a fluorescent cytokine reporter gene were stimulated to differentiate into a subset and split into three groups, in which the ability to interact with cytokines produced by other cells was manipulated. One group had no restrictions (control), the second group had restricted interactions to cytokines that should be permissive to differentiation, and the third group had restricted interactions to cytokines that should be antagonistic to differentiation. These groups were imaged for 23 hours using confocal microscopy, and the images were analyzed to monitor cytokine expression (i.e., fluorescence) and cell area as indicators of a population’s progress toward differentiation. Tests were performed to detect if there were significant differences in the mean ranks of cell fluorescence and cell area among the populations. Ultimately, this study found that the population in which antagonistic cytokinesignaling was restricted had significantly higher mean rank of both cell fluorescence (p < 2.2 x 10-16) and cell area (p < 2.2 x 10-16) relative to the other populations, indicating more progress toward differentiation. Of the three populations, cells with restriction in permissive cytokine-signaling had significantly lower mean rank of both cell fluorescence (p < 2.2 x 10-16) and cell area (p < 2.2 x 10-16) relative to the other populations, indicating less progress toward differentiation. Thus, the results of this study ultimately indicate that manipulating cytokine signals has a significant influence on Thelper cell differentiation, with cytokines acting as a likely social signal among cells. This result also provides evidence that T-helper cells may participate in collective decision-making, supporting the swarm framework proposed in Schrom and Graham 2017. Future studies should observe treated populations for extended periods of time (72 hours or longer) and investigate the influence of cytokine restrictions on differentiation to other subsets.