PMC

(A) Cytokine secretion and uptake is followed by receptor internalization, leading to a reduced cytokine concentration in the medium. (B) Schematic of the high cell-density scenario. A cytokine secreting cell is surrounded by a layer of responder cells that provide a diffusion barrier for the cytokine. Signaling can be autocrine (J_auto), i.e. cytokine molecules are bound by receptors of the cytokine secreting cell, or paracrine (J_para), i.e. bound by receptors on responder cells. (C) Cytokine concentration profile in the model with homogeneous secretion and uptake. (D) Cytokine concentration 1 μm away from the cytokine secreting cell, in the limits of high and low cell-density. (E) Paracrine signal J_para and autocrine signal J_auto (inset) as fraction of secreted cytokine molecules. (F) Autocrine and paracrine uptake in the high cell-density scenario. Dotted lines are approximations in the limit of fast diffusion (see ). Parameter values: R = 4000 mol./cell, and see .

(A) Large-scale simulation (2198 Th cells) with one IL-2 secreting Th cell placed in the center, q_eff = 10⁶ molecules/h. 71 Th cells are activated (IL-2R^high, marked gold). (B) Activated Th cells in simulations as shown in A, for varying values of q_eff. (C) Spatial range of the effective paracrine signal. Dots are replotted from panel B, with the signal range determined given in cell distances, i.e. by the distance from the center of the region to the most distant activated cell, normalized by the distance between the centers of two cells (15μm). The solid line is a best fit to the function
f(x) = a ln(x/x ₀)
(D) Distribution of traveling distances of cytokine molecules. The traveling distance is the distance from the secreting cell at which ligands are bound by receptors. These distances are obtained from a pulsed, homogeneous secretion simulation in a domain of 4913 responding cells (17 in each direction in 3D; see ). The traveling distance is shown as signal range is, calculated as in (C).

(A) Model scheme: As in , with addition of constitutively IL-2R^high Treg cells. As compared to Th cells, Treg cells have both initially higher receptor number (basal IL-2Rα expression rate) and higher IL-2 induced receptor expression. (B) Setup of the simulation, see . Here, from a total of 216 cells, again 54 are randomly chosen as secretory, and additionally, 54 are randomly chosen Treg cells. (C-D) IL-2 concentration at indicated time points, see . None of the 108 responder Th cells were activated. (E) Fraction of activated (IL-2R^high) Th cells after 30h simulation time in the in silico Th cell culture or Th-Treg coculture (see Figs and ) under various conditions. The paracrine signal increases sharply with the effective secretion rate or the fraction of cytokine secreting cells, but is almost independent of the cell-to-cell distance (shortest distance between cell surfaces).

(A) Model: Cytokine molecules are released into a cylindrical region (radius of contact area: 2μm) between Th cell and an APC or other opposed cell. Molecules can be taken up by cytokine receptors on the cytokine secreting cell (autocrine signal J_auto) or on the opposed cell (synaptic signal J_synapse), and molecules reaching the outer boundary escape (signal J_escape, ‘effective secretion rate’) and do not return. (B) Cytokine concentration in the immunological synapse (cylinder depicted in A). IL-2R^low: R = 100; IL-2R^high: R = 4000; coordinates z and r refer to the position inside the cylinder as defined in panel A. ‘Synapse’: Tight synapse with synaptic distance 20nm. ‘No synapse’: Cell-to-cell distance of 2μm, i.e. no synapse is formed. (C) Distribution of cytokine signals in the cases of IL-2R^low or IL-2R^high cytokine secreting cells, for different types of opposed cells: APCs do not express cytokine receptors (R_resp = 0), while Treg cells express high levels (R_resp = 10⁴ mol./cell) and opposed Th cells express basal levels (R_resp = 100 mol./cell) of IL-2R. Note that J_synapse does not exist if the opposed cell is an APC, and J_synapse equals J_auto in the case of a T-T synapse with two of IL-2R^low Th cells (bottom left).

(A) Model scheme: (left panel) A fraction of Th cells (about 25%) releases cytokine molecules into the immunological synapse with rate q. Most of these molecules bind to the polarized cytokine receptors on the cytokine secreting cell (see ), a smaller fraction (about 20%) escapes the synapse and is considered as effective, polarized secretion rate q_eff. (right panel) Intracellular receptor dynamics include receptor expression (rate v), binding of cytokine molecules (k_on, k_off), internalization of free and bound cytokine receptor (k_iR, k_iC), receptor recycling (k_rec) and receptor degradation (k_deg). Further, IL-2 degradation with rate k_d is considered. (B) Setup of the simulation of a T cell population in three dimensions. The cell-to-cell distance (shortest distance between cell surfaces) and cell radius are both 5μm. The simulations included 216 cells, of which 54 were randomly chosen as secretory (marked pink). Only T cells are included in the simulation, APCs are assumed to fill the region between T cells, and by that induce synapse formation leading to polarized effective secretion at randomly chosen surface points. Only non-secretory T cells express IL-2R in the simulations, because receptors of IL-2 secreting cells are considered in terms of the reduced, effective secretion rate. (C-D) IL-2 concentration at indicated time points after starting the simulation. 59 of the 162 non-secretory cells (see B) were activated, defined by expression of more than 4000 receptors after 30h simulation time (marked gold). (C) No transparency, only the surface of the cubic region is visible. (D) Limited transparency allows viewing concentration profiles inside the region. (E-F) Time course of the receptor number (E) and average IL-2 concentration at the surface (F) of activated and not activated Th cells from the simulation in C-D. Solid lines indicate averages, blurred region standard deviations, and the black line in (F) is the average IL-2 concentration in the simulated region.