Naturally occurring CD4+
Treg cells are regarded as major players in mediating peripheral tolerance to autoantigens, and thus, can prevent the onset of autoimmune diseases. In humans, this is substantiated by the high incidence of autoimmunity in patients with genetic loss of FOXP3 and the associated immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) (17
). In addition, Treg defects have also been reported in patients affected with common autoimmune disorders, including rheumatoid arthritis, multiple sclerosis, psoriasis, and type 1 diabetes (32
). Our data demonstrates that CD4+
Treg cells from WASp-deficient mice have marked defects in suppressor function in vitro. Interestingly, the defects we observe in WASp-deficient cells are somewhat more severe than those described in the very recent report by Humbalt-Baron and colleagues (36
). Moreover, our work provides evidence that the same defect is present in patients with WAS. Indeed, CD4+
Tregs from four out of five WAS patients studied also had impaired capability to suppress in vitro proliferation of target T cells. Together, these data raise the possibility that abnormal Treg cell function may contribute to the pathogenesis of autoimmune complications associated with WAS.
It has recently been shown that, unlike murine cells, activated human CD4+
T cells can transiently express FOXP3. Whether or not these activated FOXP3+
T cells have suppressor activity is a current area of debate (27
). It has also been recently reported that expression of the IL-7 receptor α chain, CD127, can discriminate between activated CD25+
T cells and CD25+
regulatory T cells (28
). Although we have not sorted for a CD127-negative population in CD4+
T cells from WAS patients and controls (due to the limiting numbers of CD4+
cells available), we have verified that our stringent selection for CD4+
cells markedly decreases the fraction of CD127+
cells among sorted populations. Moreover, sorted cells from healthy controls and WAS patients contain equal fractions of CD127+
cells. In addition, WAS patients and controls in this study showed similar frequencies of CD4+
cells (Supplementary Fig. 1
), indicating that the WAS patients examined did not have increased numbers of activated CD4+
lymphocytes. Finally, sorted Treg cells from WAS patients and healthy controls failed to proliferate when stimulated with anti-CD3 and anti-CD28 () or PHA, which can induce some proliferation of WAS T cells (Supplementary Fig. 2
), thus exhibiting anergic characteristics of true Treg cells.
The exception of normal Treg suppression activity by cells from patient WAS34, however, indicates that defective Treg function is not consistent in all WAS patients. It is possible that Treg activity may decline with time, following the reported age-dependent attrition of the T lymphocyte compartment in WAS (38
). However, it can also be hypothesized that the defect of Treg function in WAS is not complete and may be modified by other factors. Indeed, we have seen that in vitro suppression can be partially rescued by pre-activation of murine Treg cells with IL-2. However, our observations of marked defects in suppression in Treg cells from WASp-deficient mice, which completely lack protein expression and are more genetically homogeneous, strongly support that WASp is required for full Treg suppressor function.
Although we do not yet understand the mechanism(s) responsible for the defective function of WAS Treg cells, it is likely that multiple factors may contribute. Treg cell suppression function depends on TCR stimulation (40
) and requires direct cell-cell interaction (41
). WASp deficiency is associated with defective TCR-mediated activation and impaired formation of the immunological synapse (6
), both of which may affect Treg cell suppression activity. Interestingly, the partial rescue of in vitro suppression defects by pre-activation of WASp-deficient Treg cells with IL-2, suggests that optimization of T cell activation may overcome the defect of WAS Treg cells. However, other factors may also contribute to impaired negative regulatory mechanisms in WAS, including the documented defect in T cell receptor downregulation (9
), and decreased and delayed expression of CTLA-4 (45
), an important T cell inhibitory receptor. We have further found that WAS-deficient murine CD4+
T cells show impaired TCR-dependent restimulation-induced cell death (Nikolov et al., PLS, FC, RMS, manuscript submitted). The relative contribution of these potential mechanisms to the breakdown of peripheral tolerance in WAS patients remains to be established. Nevertheless, the demonstration of impaired suppressor function in Tregs from both WASp-deficient mice and WAS patients suggests that defective regulatory T cell function may be an important factor contributing to the immune dysregulation in WAS and suggests avenues for exploration of novel and improved forms of treatment for autoimmune complications of this disease.