A Transendocytosis Model of CTLA-4 Function Predicts Its Suppressive Behavior on Regulatory T Cells

Abstract

Manipulation of the CD28/CTLA-4 pathway is at the heart of a number of immunomodulatory approaches used in both autoimmunity and cancer. Whilst it is clear that CTLA-4 is a critical regulator of T cell responses, the immunological contexts in which CTLA-4 controls immune responses are not well defined. Here we show that whilst CD80/CD86-dependent activation of resting human T cells caused extensive T cell proliferation and robust CTLA-4 expression, in this context CTLA-4 blocking antibodies had no impact on the response. In contrast, in settings where CTLA-4+ cells were present as “regulators”, inhibition of resting T cell responses was dependent on CTLA-4 expression and specifically related to the number of antigen presenting cells. At low numbers of APC or low levels of ligand, CTLA-4-dependent suppression was highly effective whereas at higher APC numbers or high levels of ligand, inhibition was lost. Accordingly, the degree of suppression correlated with the level of CD86 expression remaining on the antigen presenting cells. These data reveal clear rules for the inhibitory function of CTLA-4 on Treg which are predicted by its ability to remove ligands from antigen presenting cells.

Restricting Glutamine or Glutamine-Dependent Purine and Pyrimidine Syntheses Promotes Human T Cells with High FOXP3 Expression and Regulatory Properties

“…Because CTLA-4 hi CD25 hi CD4 T cells were somewhat more strongly enriched in FOXP3 hi cells, we might have expected them to be stronger suppressors for this reason but not due to their higher CTLA-4 expression, as CTLA-4-mediated inhibition was shown to operate via cell-extrinsic mechanisms through the shared CD28/CTLA-4 ligands CD80/CD86 that were not specifically provided in our assays (44 hi CD25 hi T cells during the suppressor assays, we speculated that IL-17A might be associated with inferior inhibition of proliferation in these assays. This was not the case, however.…”

Identifying functional defects in patients with immune dysregulation due to LRBA and CTLA-4 mutations

“…16,29,30 The ability of CTLA-4 to physically capture its ligands via transendocytosis 22 from antigen-presenting cells is predictive of CTLA-4 function on Tregs. 11 Here, we used a simplified ligand uptake assay, which uses soluble CD80-Ig, to test the key features of CTLA-4 function, namely ligand binding and CTLA-4 trafficking. We have previously shown that uptake of antibodies and ligands by CTLA-4 at 37°C is a convenient measure of CTLA-4 trafficking.…”

“…6,31,32 We have been repeatedly unable to show any intrinsic effects of CTLA-4 deficiency on CD4 T-cell responses in the absence of Treg 5 and are likewise unable to demonstrate an effect of anti-CTLA-4 blockade on proliferation of CD4 Tcon, suggesting they are not subject to intrinsic CTLA-4 regulation. 11 We would urge caution in using CD4 T-cell proliferation as a measure of CTLA-4 defects because there is abundant literature showing that CTLA-4 has little intrinsic ability to directly affect these aspects of T-cell function. 15,33 In contrast, the cell-extrinsic (regulatory) function of CTLA-4 is borne out by numerous studies.…”

“…Treg testing in vitro is notoriously difficult and in vitro assays are frequently performed in ways that are uninformative for investigating CTLA-4 function. 11 Despite an understanding of the general principles of CD28 and CTLA-4 in T-cell biology, 12 the precise physiological mechanisms behind CTLA-4 function are still debated, [13][14][15] hampering the design of functional tests. Much of the biology of CTLA-4 concerns Foxp3 Tregs, 16 although it is also induced upon activation of Foxp3 2 conventional T cells (Tcons).…”

DOCK8 enforces immunological tolerance by promoting IL-2 signaling and immune synapse formation in Tregs

“…A stable IS between Tregs and DCs is important for transendocytosis (49,50), the process by which CTLA-4-expressing Tregs acquire the costimulatory molecules CD80 and CD86 from DCs, thereby decreasing the costimulatory function of DCs, rendering them tolerogenic. Following the decrease in costimulatory molecules on the DC, Tregs become increasingly motile and move on to form transient interactions with other DCs (51).…”