Mitochondrial changes synergize with long chain fatty acid derivatives to support Th17 inflammation in diabetes

Obesity-associated Type 2 diabetes (T2D) is driven by chronic inflammation. A combinatorial Th17 cytokine profile characterizes and mathematically predicts T2D in people, but the mechanisms that generate the Th17 profile are not clear. We tested the possibility that anaerobic glycolysis, which fuels inflammatory cytokine production from multiple immune cell types, drives the T2D-associated Th17 profile. We showed that activated PBMCs and purified CD4+T cells from T2D subjects prefer anaerobic glucose metabolism to produce ATP regardless of fuel availability. Unexpectedly, glucose starvation did not abrogate the T2D-predictive Th17 profile. Gene expression array suggested that mitochondrial fatty acid uptake catalyzing protein CPT1a differentiated PBMCs from T2D and ND subjects. CPT1a inhibition by etomoxir strongly down-regulated the Th17 profile, surprisingly independent of CPT1a-mediated fatty acid oxidation (OXPHOS). These data suggest that OXPHOS-independent mitochondrial changes that are also glucose-independent support the Th17 profile in T2D. Analyses from T2D vs ND PBMCs showed a lower CACT:CPT1a protein ratio in T2D, indicating defects in lipid uptake and thus defects in lipid flux. We knocked down CACT protein and overloaded PBMCs from lean subjects with fatty acylcarnitines to mimic lipid uptake defects in T2D. 16C-fatty acylcarnitine, but not 6C- or 10C-derivatives, increased frequency of CD4+IL-17+T cells, and phenocopied the Th17 profile only in CACT knockdown cells. 16C-fatty acylcarnitine alone had no effect. We conclude that excessive long chain fatty acylcarnitine combines with dysfunctional mitochondria to support a T2D-associated Th17 profile largely independent of glycolysis.

Type 1 diabetes alters lipid handling and metabolism in human fibroblasts and peripheral blood mononuclear cells

Triggers of the autoimmune response that leads to type 1 diabetes (T1D) remain poorly understood. A possibility is that parallel changes in both T cells and target cells provoke autoimmune attack. We previously documented greater Ca²⁺ transients in fibroblasts from T1D subjects than non-T1D after exposure to fatty acids (FA) and tumor necrosis factor α (TNFα). These data indicate that metabolic and signal transduction defects present in T1D can be elicited ex vivo in isolated cells. Changes that precede T1D, including inflammation, may activate atypical responses in people that are genetically predisposed to T1D. To identify such cellular differences in T1D, we quantified a panel of metabolic responses in fibroblasts and peripheral blood cells (PBMCs) from age-matched T1D and non-T1D subjects, as models for non-immune and immune cells, respectively. Fibroblasts from T1D subjects accumulated more lipid, had higher LC-CoA levels and converted more FA to CO₂, with less mitochondrial proton leak in response to oleate alone or with TNFα, using the latter as a model of inflammation. T1D-PBMCs contained and also accumulated more lipid following FA exposure. In addition, they formed more peroxidized lipid than controls following FA exposure. We conclude that both immune and non-immune cells in T1D subjects differ from controls in terms of responses to FA and TNFα. Our results suggest a differential sensitivity to inflammatory insults and FA that may precede and contribute to T1D by priming both immune cells and their targets for autoimmune reactions.

Fatty Acid Uptake Unexpectedly Supports a Th17 cytokine signature in Type 2 Diabetes (T2D) Inflammation

Metabolism controls T cell function, but the role of cellular metabolism in regulating our published T2D-associated Th17 cytokine signature is untested. Published work showed Th17 function requires fatty acid (FA) uptake, and suggested Th17s burn fat to support inflammation. We thus hypothesized that chronic exposure of immune cells to FAs, which occurs in T2D patients, drives Th17 inflammation. We used extracellular flux (XF) and cytokine analyses to show that CD3/CD28-stimulated PBMCs from BMI-matched T2D vs. non-T2D subjects were more glycolytic despite overall equal mitochondrial respiration. Consistent with this difference, T cell stimulation in glucose-free media followed by partial least squares analysis of cytokines showed glucose impacted cytokine profiles from T2D and non-T2D cells differently. To challenge the resulting prediction that PBMCs (predominantly T cells) from the two groups used different mechanisms to fuel cytokine responses, we performed XF analysis in the presence of FA +/− glucose. Non-T2D cells utilized FA as a fuel source, but T2D cells were dependent on glucose, despite data showing diabetic vs. non-T2D PBMCs store more lipid in response to elevated exogenous FA. Etomoxir, an inhibitor of FA transport into the mitochondria, caused non-T2D cells to increase glycolysis, but had no effect on T2D cells. These data confirmed that T2D cells exclusively burn glucose, not FA, to generate energy, and that T2D cells cannot be forced to use FA as a fuel. Finally, blockade of FA transport into the mitochondria revealed that FAs were associated with Th17 cytokine production. Together, our results support the unexpected conclusion that FA uptake is uncoupled from FA oxidation to drive the Th17 inflammation in T2D.

Role of innate signaling pathways in the regulation of antiviral IgG2c responses (108.9)

Long-term antibody responses provide protection from re-infection and recrudescence of persistent viruses. IgG2a/c is a predominant immunoglobulin isotype in most virus infections. Using a polyomavirus (PyV) mouse model, our lab has shown that MyD88 KO mice generate low levels of virus-specific IgG after the acute phase of infection, have reduced numbers of long-lived plasma cells and make very low levels of IgG2c, but normal levels of IgG1. Investigating the pathways upstream and downstream of MyD88, here we show that in response to PyV infection (i) Unc93B1 mutant mice, which are triple deficient in TLR3, 7 and 9 have diminished IgG2c, but normal IgG1 responses; (ii) TLR9 KO and TLR7 KO mice do not have greatly altered IgG isotype responses as observed in MyD88 KO mice or the Unc93B1 mutant mice, suggesting that these receptors play a redundant role; and (iii) mice lacking IRF5, a transcriptional regulator downstream of MyD88, have greatly diminished IgG2c and normal IgG1 responses, similar to MyD88 KO mice. The isotype switching defect observed in IRF5 KO mice is B cell intrinsic, as transfer of IRF5 KO B cells and WT T cells into RAG KO mice results in diminished antiviral IgG2c secretion compared to mice reconstituted with B6 B and T cells. Therefore, our studies suggest that TLR7/9-MyD88-IRF5 pathways in B cells play an essential role in the generation of IgG2a/c responses to virus infection.

Maintenance of serological memory in virus-infected T cell-deficient mice (105.36)

Many viruses can induce strong acute T cell-independent (TI) B cell responses due to their complex arrays of repetitive antigenic determinants on virions, coupled with the induction of innate cytokines, but T cell help is thought necessary for the development of long term humoral immunity in the form of long-lived plasma cells and memory B cells. However, we have found that T cell deficient (TCRbxd KO) mice persistently infected with polyomavirus (PyV) develop long-lasting high serum antiviral IgG responses, and we questioned whether these mice could generate TI B cell memory. TCR bxd KO PyV-infected mice had very low numbers of antibody secreting cells within the bone marrow compared to wild type (WT) mice. Moreover, by using a functional adoptive transfer assay, memory B cell responses were not detected in T cell deficient mice. In SCID mice reconstituted with TCRbxd KO mouse splenocytes, antiviral serum IgG gradually decreased, presumably due to a lack of supply of naïve B cells from bone marrow. In contrast, CD4 T cell deficient mice lack T cell help but have only short-lived antibody responses, perhaps due to control of persistent antigen by CD8 T cells. Therefore, we hypothesized that a high level of the persistent PyV antigen may subvert immunological tolerance and activate naïve B cell populations continuously, thereby maintaining the long-lasting IgG responses in T cell deficient mice. Here, we provide evidence for that hypothesis.