TOPICS

Role of protein phosphatase 2A as a regulator of inflammation

Early studies reported that the catalytic subunit of the serine/threonine phosphatase PP2A (PP2Ac) is overexpressed by T cells from patients with systemic lupus erythematosus (SLE). In order to investigate a possible pathogenic role of such defect, we generated a transgenic mouse that overexpresses PP2Ac in T cells. This mouse exhibited an abnormal susceptibility to the development of immune complex-mediated glomerulonephritis (Crispín et al., J Immunol 2012). In further studies, we demonstrated that the phenotype was caused by epigenetic modifications that facilitated the transcription of pro-inflammatory genes (e.g. Il17a) in an IRF4-dependent manner (Apostolidis et al., J Biol Chem 2013). PP2A is also an essential element in the biology of regulatory T cells because its conditional deletion in FoxP3⁺ cells abolishes their suppressive capacity and triggers a severe systemic autoimmune disease (Apostolidis et al., Nat Immunol 2016).   

Mechanisms that control the length of immune responses

In order to better understand the biology of PP2A and its contributions to the development of autoimmunity, we analyzed the expression of its regulatory subunits in human T cells. We found that B55ß is induced in activated T cells upon cytokine withdrawal and its presence is necessary and sufficient for the T cells to undergo apoptosis induced by IL-2 withdrawal (Crispín et al., Proc Natl Acad Sci USA 2011). By analyzing the kinetics of apoptosis and B55ß expression in T cells from patients with SLE, we identified a group of patients whose T cells are resistant to the induction of apoptosis during cytokine withdrawal. Importantly, IL-2 deprivation did not induce B55ß upregulation in these patients suggesting that abnormalities in its expression may contribute to autoimmunity by allowing the survival of activated T cell clones.    

CD8 downregulation as a mechanism of peripheral tolerance

Patients and mice with lupus frequently have increased numbers of TCR-αß⁺ double negative (DN) T cells. DN T cells are a normally scarce T cell subpopulation that lacks the expression of the CD4 and CD8 coreceptors. In initial studies, we reported that DN T cells can produce pro-inflammatory cytokines (i.e. IL-17 and IFN-gamma) and infiltrate target organs of patients with SLE (Crispín et al., J Immunol 2008). Later, we reported that DN T cells are generated by loss of CD8 that occurs after the activation of CD8⁺ T cells (Crispín et al., J Immunol 2009). In order to understand the factors that govern CD8 to DN conversion, we used an in vivo adoptive transfer model. We learned that when CD8⁺ T cells are productively activated, as in the context of an infection, expression of CD8 is maintained. However, when CD8⁺ T cells encounter cognate antigen presented as self, they undergo an inactivation process that includes the downregulation of CD8 and the expression of high levels of inhibitory molecules including PD-1 and the transcription factor Helios (Rodríguez-Rodríguez et al.,

Biological effects of risk alleles associated to autoimmune diseases

Genome-wide association studies (GWAS) have identified a relatively large number of genes and loci associated to SLE and other autoimmune diseases. These analyses detect common single nucleotide polymorphisms (SNP) whose frequency is significantly higher in patients than in healthy controls. One of the SNPs that is thought to confer a higher risk for the development of SLE is located in ITGAM, the gene that encodes the alpha chain of the adhesion molecule Mac-1. The risk allele of ITGAM represents a non-synonymous mutation that alters the protein (R77H). In order to determine the effects of the R77H substitution, we generated cells lines expressing the wild type or the risk variant of Mac-1. In vitro studies revealed that the R77H variant diminishes the ligand-binding capacity of Mac-1, in particular under shear flow (Rosetti et al, J Immunol 2012). Further studies demonstrated that the risk allele is unable to form “catch bonds”, a molecular mechanism that allows integrins to bind with high affinity (Rosetti et al, Cell Reports 2015). To evaluate the in vivo consequences of decreased Mac-1 function, we analyzed mice that lack Mac-1 and express the human FcgRIIA. Intravenous injection of immune complex rich sera obtained from patients with SLE caused the glomerular deposit of large amounts of immune complexes. Strikingly, absence of Mac-1 was associated to a dense infiltration of neutrophils and to the development of severe glomerulonephritis. Based on these, we have proposed that Mac-1 regulates neutrophil activation in response to immune complexes. The SLE-associated R77H variant of Mac-1 contributes to disease by allowing an amplified neutrophil activation that leads to the instigation of disproportionate tissue injury (Rosetti et al, J Immunol 2012).

 

Role of Helios Transcription Factor as New Cancer-Associated Immunosuppressant and Strategies to Inhibit It