Date Published: April 5, 2019
Publisher: Public Library of Science
Author(s): Bettina Wanko, Matteo Tardelli, Alexander Jürets, Angelika Neuhofer, Gerhard Prager, John Morser, Lawrence L. Leung, Günther Staffler, Maximilian Zeyda, Thomas M. Stulnig, Susan R. Rittling.
T cells are crucial players in obesity-mediated adipose tissue inflammation. We hypothesized that osteopontin (OPN), an inflammatory protein with enhanced activity when proteolytically cleaved, affects the number of viable T cells in adipose tissue and assessed inhibition of the interaction between T cells and thrombin and matrix metalloproteinases-cleaved OPN using antibodies and postimmune sera. Gene expression of T cell markers in adipose tissue from wild-type (wt) and Spp1-/- (OPN deficient) mice was analyzed after 16 weeks of high fat diet (HFD) or low fat diet (LFD) feeding. CD3, CD8 and OPN gene expression in omental adipose tissue from individuals with obesity was measured. OPN-T cell interactions were assessed with a fluorescence-based adhesion assay and blocked with antibodies targeting OPN. Comparison of T cell gene expression in adipose tissue from wt and Spp1-/- mice showed that OPN affected the number of T cells while in humans, levels of OPN correlated with T cell markers in omental adipose tissue. The interaction between T cells and cleaved OPN was blocked by postimmune sera following OPN peptide vaccinations and with monoclonal antibodies. In conclusion, levels of OPN affected the number of T cells in obesity and antibodies against cleaved OPN antagonize OPN-T cell interactions.
T cells, mostly Th1 cells  as well as CD8+ T cells (cytotoxic T cells) , play an important role in obesity-mediated adipose tissue inflammation as they infiltrate adipose tissue at an early stage of inflammation[1–3]. Interferon γ (IFNγ), secreted by Th1 and CD8+ T cells, triggers the polarization of macrophages towards a M1 phenotype while the Th2-secreted cytokines IL-4 and IL-13 induce a shift towards a M2 phenotype. Depletion of CD8, either by genetic ablation or antibodies, reduces the number of macrophages in adipose tissue while increasing insulin sensitivity . Passive vaccination with an anti-CD3 antibody or its F(ab’)2 fragment improves obesity-induced insulin resistance and reduces the number of M1 type macrophages in adipose tissue [1,5]. Hence, T cells are crucially involved in the initiation of obesity-driven adipose-tissue inflammation and its metabolic sequelae.
Because OPN increases the number of macrophages in adipose tissue in obesity , we hypothesized that OPN might also influence the number of T cells, which precede macrophage accumulation in adipose tissue. To this end, we compared the level of T cells in adipose tissue from wt and Spp1-/- obese mice. Spp1-/- mice on LFD were not included in this study as they do not show any systemic or adipose tissue related change compared to wt mice on LFD. Also both plasma inflammatory cytokine levels and inflammatory cytokine mRNA expression in adipose tissue are the same in wt mice and Spp1-/- mice on LFD. [6,16]. The significant increase in adipose tissue gene expression of the T cell markers, CD3, CD4 and CD8, in wt mice indicated T cell enrichment in adipose tissue in obesity confirming previous studies [1–3]. In contrast, OPN deficient Spp1-/- mice had reduced T cell marker gene expression demonstrating that presence of OPN is critical for T cell accumulation in obese adipose tissue. We could further corroborate this result by staining for CD3, CD4 and CD8 markers in adipose tissue by immunofluorescence where we found that the signal intensity in wt mice fed HFD was significantly higher than in HFD Spp1-/- mice suggesting that OPN is implicated in the increased number of T cell numbers in adipose tissue.