Our data show T2D patients have an increase in the proinflammatory Th17/Th1 axis that is supported by monocytes, a cell type more traditionally associated with inflammation in T2D. This conclusion is supported by the increase in circulating Th17 cells and elevated secretion of IL-17 and IFN-γ by T cells from T2D patients. These findings are independently confirmed by an increase in molecular signatures that characterize Th17 cells, including active structures at the IL-17 and RORC promoters in fresh ex vivo T cells and in T cells incubated in the context of PBMCs from T2D patients. Increased proinflammatory T cell function is compounded by the decrease of the CD4+ Treg compartment. Importantly, our data show for the first time, to our knowledge, a loss in homeostasis in T cell subset balance that most likely promotes the chronic inflammation characterizing T2D patients. Our findings also identify a new role for monocytes in T2D: support of proinflammatory T cell function.
Several groups have recently identified important roles for T cells in adipose tissue inflammation and insulin resistance in a mouse model of T2D. Predictably, inflammatory IL-17+
cells are elevated in at least some adipose depots of obese/insulin-resistant mice (26
). Additionally, previous work shows that IFN-γ downregulates the insulin response in adipocytes (54
), thus identifying a mechanism that links proinflammatory T cell function to a more traditional diabetogenic cell type. Complementary studies showed Tregs play important roles in inhibiting insulin resistance, and loss of Tregs promoted inflammation (26
). Our data critically extend the conclusions of these animal studies into humans and demonstrate that a focus on T cells represents a clinically important area of T2D research. Although our new studies did not specifically define a role for Th1 and Th17 cells in adipose tissue inflammation, the data provide evidence for a role of IL-17– and IFN-γ–producing T cells in systemic T2D inflammation, which likely parallels adipose-associated events.
Multiple pieces of evidence indicate a potential proinflammatory feed-forward loop between adipocytes and T cells in obesity and T2D. Over nutrition and obesity induce adipose tissue to secrete proinflammatory factors such as IL-6, a cytokine that induces Th17 differentiation (33
). Increased secretion of IL-6, along with IL-1β and TGF-β (29
) can lead to systemic inflammation and a pro-Th17 skewing milieu, which in turn results in elevated levels of IL-17. T2D patients also have elevated levels of serum IL-12 (57
), a cytokine that promotes Th1 differentiation and elevated IFN-γ production (59
). Conversely, adipocytes are potential targets of proinflammatory T cell cytokines. Adipocytes express significant levels of the IL-17 receptors IL-17RA and IL-17RC (63
) and respond to IL-17 by secreting IL-6, which may reinforce the IL-6 produced under hypercaloric conditions (63
). Similarly, adipocytes respond to IFN-γ by attenuating JAK/STAT activation, hence insulin signaling (54
), which leads to insulin resistance. Taken together, these studies support the possibility that adipocytes and T cells synergize to establish a feed-forward loop of chronic inflammation. Additional work on the specific roles of Th1 and Th17 cells in exacerbating chronic inflammation (perhaps via adipocytes) would further our understanding of the complex interplay between two seemingly unrelated cell types in T2D.
Two chemokine receptors, CCR4 and CCR6, identify IL-17–producing memory Th17 cells (35
). Later reports found CCR4 and CCR6 on the surface of Tregs as well, although these were primarily in the tonsil (64
), which, by its nature, is chronically inflamed when excised from patients. Based on the reciprocal relationship between Th17 cells and Tregs, it is possible that CCR4 and CCR6 could be yet another set of features that these functionally opposed cell types share, at least under some conditions. Importantly, elevated secretion of IL-17 in T2D patients independently confirmed conclusions from the initial CCR4/CCR6 studies, regardless of whether these chemokine receptors uniquely identified Th17 cells in T2D patients. An alternative explanation for the elevated percentages of CCR4+
cells, along with elevated secretion of IL-17, is that in the inflammatory milieu found in T2D patients, Foxp3+
cells could be induced to produce IL-17. Foxp3/IL-17A double-positive T cells identified by published studies on ND T cells (65
) demonstrate that generally anti-inflammatory Tregs may have proinflammatory activities under some conditions. CCR4/6 expression on Tregs and Th17 cells may explain the seeming contradictory findings that the percentage of CCR4+
T cells, but not IL-17 secretion, positively correlates with age (, Supplemental Fig. 2
). Likewise, IL-17 secretion but not the percentage of CCR4/6 T cells correlated with T2D severity as measured by HbA1c (, Supplemental Fig. 2
). Future studies will identify the mechanistic underpinnings of these related but biologically discernible cell populations.
Chemokine receptors such as CCR4 and CCR6 are critical for recruitment of appropriate immune cells to sites of inflammation. Preliminary mRNA analysis indicates expression of CCL20, the ligand for CCR4, in human adipose-associated stromal vascular fractions (S. Fried and D. Gong, unpublished observations). Interestingly, because Tregs and Th17 cells share trafficking receptors, both cell types could be recruited to adipose tissue. These findings raise the possibility that Tregs recruited to a pro-Th17 milieu in the adipose tissue through CCR4 and CCR6 are subsequently skewed toward the Th17 phenotype. The plasticity between Tregs and Th17 cells indicate a potential for phenotypic changes based on signals from the environment, which in a non-disease state would be beneficial for a balanced immune response, but highly detrimental when homeostasis is lost.
Because inflammatory cytokines induce insulin resistance, reducing inflammation is an important outcome for T2D treatments. Our data indicate that one potential approach toward this goal would be to reset the balance between the proinflammatory and anti-inflammatory T cell subsets, Th1/Th17 and Tregs, respectively, perhaps by identifying factors that drive BMI-mediated elevations in CCR4+CCR6+ T cells that are present in T2D donors but absent in ND donors (). Alternatively, identifying mechanisms that explain BMI-dependent decreases in Tregs from T2D samples () may offer therapeutic options. Finally, our work shows monocyte manipulation is a viable indirect method for altering T cell function in T2D. An intimate understanding of the molecular mechanisms behind the reciprocal Th17/Treg relationship is essential to be able to safely manipulate T cell subset balance to re-establish homeostasis in a disease environment. Targeting the imbalance in T cells as a treatment for T2D patients could be used as an Achilles heel to reduce inflammation leading to improved glucose tolerance and reduced insulin resistance.