The directed secretion of cytokines into the immunological synapse between T cells and antigen-presenting cells may also contribute to confine cytokines (37). Under conditions of limited IL-2 secretion, our results indicate that IL-2 signaling from Th cells to Treg cells takes place in microenvironments (e.g., between cells bound to the same antigen-presenting cell). can compete for IL-2 and restrict its range of action through efficient cellular uptake. Depending on activation status and spatial localization of the cells, IL-2 may be consumed exclusively by Treg or Th cells, or be shared between them. In particular, a Treg cell can deprive a stimulated Th cell of its IL-2, but only when the cells are located in close proximity, within a few tens of micrometers. The present findings explain how IL-2 can play two disctinct 20-HEDE functions in immune regulation and point to a hitherto largely unexplored spatiotemporal complexity of cytokine signaling. 100 m) and IL-2R expression (h) allowed us to apply a quasisteady state approximation for diffusion and reduce the model to coupled regular differential equations (QSSA model; and Fig. S3). Thus the model predicts that this IL-2 secretion rate must exceed a threshold value to switch IL-2R expression to the activated state and permit considerable autocrine IL-2 signaling. Open in a separate windows Fig. 2. Digital IL-2R expression in Th cells. (to to = 10 m), the Treg cell functions 20-HEDE as a potent sink for IL-2, and the paracrine IL-2 transmission causes further upregulation of IL-2R around the Treg cell (Fig. 4= 10 m). The Th-cell activation threshold is usually increased and bi-stability enhanced (black collection) (Fig. 2). By contrast, the upregulation of IL-2Rs around the Treg cell is usually practically continuous (red collection). (to for further explanation). In contrast to the Th-cell dynamics, the Treg-cell response curve does not show a measurable effect of bistability (although a small hysteresis loop is present). Treg cells escape the digital opinions switch because high-affinity IL-2Rs are already expressed in the resting state. Therefore, IL-2R expression in Treg cells will be a more graded function of Th-cell IL-2 secretion (Fig. 4is the IL-2 diffusion coefficient, and = 10 m2s?1 and test): 0.017 (2.5 10?3 g/mL OVA), 0.002 (0.01 g/mL), 0.076 (0.05 g/mL), and 0.359 (1 g/mL). To characterize the IL2R expression patterns in Th cells and Treg cells, we fitted the IL-2R histograms by the sum of two log-normal distributions, capturing the cells with basal and activated IL-2R expression, respectively (and Fig. S4). The activated Th cells experienced a constant mean IL-2R expression independent of the antigen stimulus (Fig. 6and ?and6and em G /em ). Similarly, addition of IL-2 to a TregCTh coculture restored proliferation of Th cells (17) (Fig. S6). In summary, the adaptation of IL-2R expression to ambient IL-2 concentration maintained a strong competitive advantage of Treg cells for IL-2 uptake. The doseCresponse for the suppressive action of Treg cells agreed with the predicted end result of IL-2 competition, which is effective at moderate but not high rates of IL-2 secretion. Conversation The spatiotemporal dynamics of the IL-2 network explained here have several functional implications discussed in the following (and summarized in Table S2). It has previously been proposed that a T cell stimulus must exceed a discrete threshold to trigger proliferation (1, 30). The IL-2R switch found here provides a mechanistic basis for this activation threshold. Digital regulation in T cells also occurs in antigen transmission transduction (31), recently demonstrated to be based on bistable feedback regulation of SOS (32), and in NFAT nuclear translocation (21). The autocrine IL-2 loop may have an integrative function because it requires that this activation signal Rabbit Polyclonal to OR51B2 has already exceeded the thresholding devices in intracellular signaling. In agreement with this, we observed that cell proliferation correlated with a switch to high IL-2R expression. Notably, autocrine positive opinions also occurs with other cytokines (33, 34) or growth factors (35),.The activated Th cells had a constant mean IL-2R expression independent of the antigen stimulus (Fig. efficient cellular uptake. Depending on activation status and spatial localization of the cells, IL-2 may be consumed exclusively by Treg or Th cells, or be shared between them. In particular, a Treg cell can deprive a stimulated Th cell of its IL-2, but only when the cells are located in close proximity, within a few tens of micrometers. The present findings explain how IL-2 can play two disctinct functions in immune regulation and point to a hitherto largely unexplored spatiotemporal complexity of cytokine signaling. 100 m) and IL-2R expression (h) allowed us to apply a quasisteady state approximation for diffusion and reduce the model to coupled regular differential equations (QSSA model; and Fig. S3). Thus the model predicts that this IL-2 secretion rate must exceed a threshold value to switch IL-2R expression to the activated state and permit considerable autocrine IL-2 signaling. Open in a separate windows Fig. 2. Digital IL-2R expression in Th cells. (to to = 10 m), the Treg cell functions as a potent sink for IL-2, and the paracrine IL-2 transmission causes further upregulation of IL-2R around the Treg cell (Fig. 4= 10 m). The Th-cell activation threshold is usually increased and bi-stability enhanced (black collection) (Fig. 2). By contrast, the upregulation of IL-2Rs around the Treg cell is usually practically continuous (red collection). (to for further explanation). In contrast to the Th-cell dynamics, the Treg-cell response curve does not show a measurable effect of bistability (although a small hysteresis loop is present). Treg cells escape the digital opinions 20-HEDE switch because high-affinity IL-2Rs are already expressed in the resting state. Therefore, IL-2R expression in Treg cells will be a more graded function of Th-cell IL-2 secretion (Fig. 4is the IL-2 diffusion coefficient, and = 10 m2s?1 and test): 0.017 (2.5 10?3 g/mL OVA), 0.002 (0.01 g/mL), 0.076 (0.05 g/mL), and 0.359 (1 g/mL). To characterize the IL2R expression patterns in Th cells and Treg cells, we fitted the IL-2R histograms by the sum of two log-normal distributions, capturing the cells with basal and activated IL-2R expression, respectively (and Fig. S4). The activated Th cells experienced a constant mean IL-2R expression independent of the antigen stimulus (Fig. 6and ?and6and em G /em ). Similarly, addition of IL-2 to a TregCTh coculture restored proliferation of Th cells (17) (Fig. S6). In summary, the adaptation of IL-2R expression to ambient IL-2 concentration maintained a strong competitive advantage of Treg cells for IL-2 uptake. The doseCresponse for the suppressive action of Treg cells agreed with the predicted end result of IL-2 competition, which is effective at moderate but not high rates of IL-2 secretion. Conversation The spatiotemporal dynamics of the IL-2 network explained here have several functional implications discussed in the following (and summarized in Table S2). It has previously been proposed that a T cell stimulus must exceed a discrete threshold to trigger 20-HEDE proliferation (1, 30). The IL-2R switch found here provides a mechanistic basis for this activation threshold. Digital regulation in T cells also occurs in antigen transmission transduction (31), recently demonstrated to be based on bistable feedback regulation of SOS (32), and in NFAT nuclear translocation (21). The autocrine IL-2 loop may have an integrative function because it requires that this activation signal has already exceeded the thresholding devices in intracellular signaling. In agreement with this, we observed that cell proliferation correlated with a switch to high IL-2R expression. Notably, autocrine positive opinions also occurs with other cytokines (33, 34) or growth factors (35), so that digital-switch mechanisms may be more common in cytokine signaling. Other regulatory mechanisms, such as cross-inhibition, can also convert graded input into digital output (36). The IL-2-mediated activation switch of Th cells is not cell autonomous because IL-2 diffuses. However, the model indicates that paracrine IL-2 signaling (to Treg cells or nonsecreting Th cells) 20-HEDE is limited to the neighborhood of IL-2-secreting cells, because IL-2 uptake is very efficient once IL-2R becomes upregulated. High IL-2 concentrations (0.1C1 nM) are predicted in microenvironments near secreting cells (compared with the IL-2R em K /em d of 10 pM), whereas IL-2 concentrations in supernatants of T cell cultures are much lower (pM).