Data gathered from experimental and clinical immune intervention studies point to the necessity of combination therapy to successfully cure type 1 diabetes. This could be achieved by simultaneously targeting different key immune players, such as antigen-presenting cells and T cells (pathogenic and/or Tregs), and by administering drugs that improve beta cell function/mass together with immune modulators. Such a strategy has been shown to be successful as the combination of exendin-4, a glucagon-like peptide receptor agonist, with CD3 monoclonal antibodies or anti-lymphocytes serum improved diabetes remission rates in NOD mice by enhancing recovery of residual beta cells as well as insulin content and release from beta cells [1, 2].
In the present study, we took advantage of the well-described protective effect of givinostat, a HDACi, on pancreatic beta cells. Indeed, in mouse, rat and human islets and in beta cell lines, givinostat prevents cytokine-induced beta cell death and preserves beta cell function by reducing the expression of the IFNγ-inducible chemokines CXCL9 and CXCL10, expression of nitric oxide synthase (iNOS) and production of nitric oxide through the inhibition of NF-κB transcriptional activity and extracellular signal-regulated kinase (ERK) phosphorylation [11, 12, 30,31,32]. Accordingly, we observed that givinostat could, by itself, improve glucose tolerance in recent-onset diabetic NOD-huCD3ε mice, which is probably related to the combined anti-inflammatory effects exerted at both the immune cell and islet levels [12] and/or to an increased sensitivity to insulin. Indeed, givinostat, like most agents that inhibit HDAC, has demonstrated potent anti-inflammatory properties both in vitro and in vivo. Givinostat inhibited the production of proinflammatory cytokines by concanavalin A-stimulated splenocytes or by lipopolysaccharide (LPS)-stimulated peritoneal macrophages and reduced LPS-induced in vivo systemic inflammation [9, 11, 12]. Similarly, we found that ex vivo intraislet secretion of IL-1β, IL-6 and TNF-α by immune cells was decreased in NOD-huCD3ε mice treated with oral givinostat; in contrast to IL-10, which was increased. Interestingly, Il10 gene expression was increased in the spleen of NOD mice treated with another pan-HDACi (vorinostat) from weaning [12]. The capacity of HDAC inhibitors to promote IL-10 while inhibiting inflammatory cytokines was recently confirmed in vitro on epithelial, fibroblast and myogenic cell lines, as well as in vivo in response to silicone breast implants in mice [33]. Although the role of IL-10 in diabetes remission remains to be determined, our results suggest that pancreatic islets switched from a proinflammatory to an anti-inflammatory microenvironment upon givinostat treatment.
Our previous work suggests that givinostat does not exert its actions at the histone level as we did not detect any effects of HDACi treatment on histone acetylation in NOD mice [12]. In addition, we found that givinostat reduced beta cell and peritoneal macrophage inflammatory gene expression, contrary to the dogmatic view that histone hyperacetylation leads to an open chromatin structure and transcriptional activation [12]. Transcriptomic analysis of HDACi-treated beta cells and macrophages strongly pinpoints NF-κB as the key regulating node and we found that the NF-κB subunit p65 was hyperacetylated in beta cells exposed to givinostat [12]. This hyperacetylation prevented p65 from binding inflammatory gene promoters providing a mechanistic explanation for the above observations. The anti-inflammatory effects of HDACi have been demonstrated in vitro in model systems using either insulin-producing cells, isolated islets devoid of immune cells or purified leukocytes, strongly indicating that the HDACi anti-diabetic action is exerted at both the immune system and islet level [10,11,12, 30, 31, 34].
Although oral givinostat efficiently targeted islet inflammation and promoted beta cell function, it was not sufficient to reverse established disease in NOD-huCD3ε mice. Sustained diabetes remission was achieved when givinostat was combined with a short course of subtherapeutic doses of humanised CD3 antibodies (otelixizumab). Such a combination regimen favoured pancreatic beta cell survival, secretory function and resistance to inflammation. It improved the overall metabolic status of treated mice that recovered their capacity to secrete increased amounts of insulin, control hyperglycaemia and sustain glucose tolerance.
In addition, our data suggest that givinostat treatment improved responses to CD3 antibody immune therapy. Administration of subtherapeutic doses of otelixizumab combined with givinostat drastically reduced CD8+ T cell IFNγ responses towards the immunogenic PI15-23 and IGRP206-214 peptides, and very low frequencies and absolute numbers of CD8+ T cells specific for these peptides were detected in the pLNs compared with untreated diabetic NOD-huCD3ε mice. Interestingly, as already described [20], responsiveness towards PI (but not IGRP) is sustained after CD3 antibody monotherapy; NOD-huCD3ε mice treated with otelixizumab alone still display significant numbers of PI-specific CD8+ T cells and potent IFNγ responses specific to PI. Thus, the combination with givinostat contributed to reduce tissue inflammation and provided a more complete and sustained downregulation of autoreactive T cell responses. Such an impact on autoimmune responses was not reported when CD3 antibodies were combined with exendin-4 [2]. Consistent with recent publications from our group and others, this unresponsiveness may be associated with the presence of anergic or exhausted-like T cells expressing inhibitory receptors, such as programmed death 1 (PD-1)/programmed death-ligand 1 (PD-L1)/LAG-3/T cell immunoreceptor with Ig and ITIM domains (TIGIT), as well as the transcription factor eomesodermin (Eomes) [35,36,37]. Another field of investigation concerns the impact of the combination therapy on glucose metabolism as CD3 antibodies have been shown to downregulate the expression of components of the glycolysis pathway, such as the glucose transporter GLUT1, in effector T cells and HDACi are able to inhibit GLUT1-mediated glucose transport [37, 38].
In our model, we did not notice any significant effect of givinostat on CD4+FOXP3+ Tregs. Absolute numbers and proportions are similar in untreated and givinostat-treated NOD-huCD3ε mice. We and others have previously demonstrated that FOXP3+ Tregs are preserved from the CD3 antibody depleting effect, which mostly targets activated effector CD4+ and CD8+ T cells [18, 27, 28, 37]. Consequently, Treg frequency in the CD4+ T cell compartment increases after CD3 antibody therapy. This is what we observed in the present study 1 month after treatment with otelixizumab alone or in combination; however, absolute Treg numbers were constant. By 2 months (i.e. a time point where the T cell compartment has fully reconstituted), Treg proportions and absolute numbers were similar in all groups, showing that givinostat did not further promote Treg expansion in contrast to what was reported when it was administered at weaning [12]. Such a difference may be related to the therapeutic window. We also did not find any evidence in the spleen or pLNs for the presence of Tr1 cells, another regulatory CD4+ T cell subset induced in response to IL-10 and that act in an IL-10-dependent manner [29, 39, 40]. The possible and preferential induction of Tr1 cells in the small intestine of treated mice remains to be investigated.
Therefore, our data indicate that givinostat and otelixizumab acted in synergy to restore immune tolerance within pancreatic islets where, complementary to otelixizumab-mediated elimination of pathogenic effector T cells, givinostat favoured beta cell survival and recovery of secretory function in a non-inflammatory environment. A continuous supply of givinostat was not required to sustain the therapeutic effect, suggesting a critical role in the early phase of tolerance induction at the time of CD3 antibody therapy.
In terms of clinical translation, the combination of an HDACi and CD3 antibodies offers several advantages. First, humanised CD3 antibodies have been tested in individuals with type 1 diabetes. Promising results have been obtained in phase II and III trials in terms of preservation of the insulin secretory capacity, resulting in reduced insulin need [21,22,23,24]. Second, givinostat is administered orally and its therapeutic effect is currently being evaluated in a wide range of indications. A phase II study in active systemic onset juvenile idiopathic arthritis showed that a low oral dose of givinostat achieved significant reductions in joint and systemic inflammation with no organ toxicity [41]. In addition, there is a growing interest in the use of an isoform-selective HDACi, which may have a more tailored and safer effect than a pan-HDACi. HDAC3 inhibition, using siRNA knockdown or pharmacological agents, was shown to be effective for protecting pancreatic beta cells from cytokine-induced apoptosis and improved both islet size and insulin sensitivity and secretion without haematological adverse effects [34, 42,43,44,45]. Accordingly, conditional HDAC3 ablation in beta cells of adult mice increased insulin secretion and improved glucose metabolism, supporting the finding that HDAC3 is a major regulator of gene transcription in beta cells [46]. Furthermore, HDAC7 may also be a potential target as a recent study showed that HDAC7 was overexpressed in the islets of individuals with type 2 diabetes, and this correlated with impaired insulin content and secretion [47]. HDAC7 inhibition restored glucose-stimulated insulin production in an HDAC7-overexpressing beta cell line [47].
In conclusion, our data strengthen the therapeutic potential of a small molecule HDACi in resolving chronic autoimmune inflammation in type 1 diabetes and further consolidating the therapeutic efficacy of combination treatments that simultaneously target T cells and protect pancreatic beta cells.