Antonio La Cava

The weight of leptin in immunity

Leptin is an adipocyte-derived hormone/cytokine that links nutritional status with neuroendocrine and immune functions. As a hormone, leptin regulates food intake and basal metabolism, and is sexually dimorphic — that is, its serum concentration is higher in females than in males with a similar body fat mass. As a cytokine, leptin can affect thymic homeostasis and the secretion of acute-phase reactants such as interleukin-1 and tumour-necrosis factor. Similar to other pro-inflammatory cytokines, leptin promotes T helper 1 (TH1)-cell differentiation and can modulate the onset and progression of autoimmune responses in several animal models of disease. Here, we review the advances and controversy for a role of leptin in the pathophysiology of immune responses.

Leptin surge precedes onset of autoimmune encephalomyelitis and correlates with development of pathogenic T cell responses

In the work presented here, we explored the influence of leptin on the kinetics of experimental autoimmune encephalomyelitis (EAE) onset, in the EAE-associated inflammatory anorexia, and in the development of pathogenic T cell responses. We found that the expression of serum leptin increased before the clinical onset of EAE in disease-susceptible C57BL/6J (H-2(b)) and SJL/J (H-2(s)) strains of mice, which are models of chronic-progressive and relapsing-remitting EAE, respectively. This increase in serum leptin correlated with disease susceptibility, reduction in food intake, and decrease in body weight. Indeed, acute starvation, which is able to prevent the increase in serum leptin, delayed disease onset and attenuated clinical symptoms by inducing a T helper 2 cytokine switch. Furthermore, immunohistochemical analysis revealed a parallel in situ production of leptin in inflammatory infiltrates and in neurons only during the acute/active phase of both chronic-progressive and relapsing-remitting EAE. We also found that leptin secretion by activated T cells sustained their proliferation in an autocrine loop, since antileptin receptor antibodies were able to inhibit the proliferative response of autoreactive T cells in vitro. Given that leptin appears to regulate EAE susceptibility, inflammatory anorexia, and pathogenic T-cell immune function, we postulate that it may offer a potential target in the treatment of multiple sclerosis.

Organ-specific features of natural killer cells

Natural killer (NK) cells can be swiftly mobilized by danger signals and are among the earliest arrivals at target organs of disease. However, the role of NK cells in mounting inflammatory responses is often complex and sometimes paradoxical. Here, we examine the divergent phenotypic and functional features of NK cells, as deduced largely from experimental mouse models of pathophysiological responses in the liver, mucosal tissues, uterus, pancreas, joints and brain. Moreover, we discuss how organ-specific factors, the local microenvironment and unique cellular interactions may influence the organ-specific properties of NK cells.

Balancing susceptibility to infection and autoimmunity: a role for leptin?

The immune responses to many infections have long been known to share features with autoimmune responses. In particular, both types of response are typified by the enhanced reactivity of T helper 1 cells - with high levels of interleukin-2, interferon gamma and tumor necrosis factor alpha - and are accompanied often by organ-specific and/or systemic damage and the production of IgG. Paradoxically, the geographical distributions of incidence of infectious diseases and autoimmunity are complementary, rather than coincident. In less-developed societies, an epidemiological association between susceptibility to infection and malnutrition has been observed, whereas in affluent countries, an increased incidence of autoimmune diseases has been described. We suggest that these observations can be explained partly by taking into consideration the immune effects of the adipocyte-derived hormone leptin, which has been shown recently to act as a link between nutritional status and the immune response.

Pancreatic expression of interferon-gamma protects mice from lethal coxsackievirus B3 infection and subsequent myocarditis.

Cardiovascular disease is one of the leading causes of death worldwide, and has been associated with many environmental risk factors. Recent evidence has indicated the involvement of pathogens such as viruses as causative agents, and specifically identified the coxsackievirus B serogroup as the leading culprit. Not only has coxsackievirus B3 (CB3) been identified from patients with cardiovascular disease, but also infection of mice with CB3 strains can reproduce human clinical heart disease in rodents. Several mechanisms have been proposed in an attempt to distinguish between pathology mediated by direct viral destruction of cardiac muscle cells or by the virus-induced immune response directed at infected myocytes or at ‘mimicked’ epitopes shared between viral and cardiac antigens. To distinguish between these mechanisms, we infected a unique mouse that diminishes the extent of infection and spread of the virus, but allows complete immunity to the virus. Transgenic mice expressing interferon-γ in their pancreatic β cells failed to develop CB-3-induced myocarditis. This work challenges the idea of the function of the immune response and ‘molecular mimicry’ in the CB-3-induced autoimmune myocarditis model, and instead favors the idea of virus-mediated damage. These results emphasize the benefit of reducing the level of viremia early during infection, thereby reducing the incidence of virus-mediated heart damage and autoimmunity.

Unraveling the multiple roles of leptin in inflammation and autoimmunity

Initially described as an antiobesity hormone, leptin has subsequently been shown also to influence hematopoiesis, thermogenesis, reproduction, angiogenesis, and immune homeostasis. Leptin links nutritional status and proinflammatory T helper 1 immune responses, and the decrease in leptin plasma concentration during food deprivation leads to impaired immune function. This review focuses on the multiple roles of leptin in chronic inflammation and autoimmunity and suggests new possible therapeutic implications for leptin modulators.

Cutting Edge: Regulatory T Cells Directly Suppress B Cells in Systemic Lupus Erythematosus

In systemic lupus erythematosus (SLE), adaptive CD4+CD25+Foxp3+ regulatory T cells (Tregs) suppress Th cells that help autoantibody (autoAb)-producing B cells. It is not known whether naturally occurring Tregs can directly suppress B cells in SLE without an intermediate suppression of Th cells. This aspect is important for its implications in the natural course of SLE, because most if not all of the clinical and pathologic effects in SLE are associated with a dysregulated production of autoAbs. In this study, we show that natural Tregs can inhibit B cell activity in vitro and in vivo in SLE through cell contact-mediated mechanisms that directly suppress autoAb-producing B cells, including those B cells that increase numerically during active disease. These results indicate that one way by which natural Tregs attempt to limit humoral autoimmunity in SLE is by directly targeting autoreactive B cells.

Central nervous system (CNS)–resident natural killer cells suppress Th17 responses and CNS autoimmune pathology

Natural killer (NK) cells of the innate immune system can profoundly impact the development of adaptive immune responses. Inflammatory and autoimmune responses in anatomical locations such as the central nervous system (CNS) differ substantially from those found in peripheral organs. We show in a mouse model of multiple sclerosis that NK cell enrichment results in disease amelioration, whereas selective blockade of NK cell homing to the CNS results in disease exacerbation. Importantly, the effects of NK cells on CNS pathology were dependent on the activity of CNS-resident, but not peripheral, NK cells. This activity of CNS-resident NK cells involved interactions with microglia and suppression of myelin-reactive Th17 cells. Our studies suggest an organ-specific activity of NK cells on the magnitude of CNS inflammation, providing potential new targets for therapeutic intervention.

Differential Effects of IL-21 during Initiation and Progression of Autoimmunity against Neuroantigen

The cytokine IL-21 is closely related to IL-2 and IL-15, a cytokine family that uses the common γ-chain for signaling. IL-21 is expressed by activated CD4+ T cells. We examined the role of IL-21 in the autoimmune disease experimental autoimmune encephalomyelitis (EAE), an animal model for human multiple sclerosis. IL-21 administration before induction of EAE with a neuroantigen, myelin oligodendrocyte glycoprotein peptide 35-55, and adjuvant enhanced the inflammatory influx into the CNS, as well as the severity of EAE. Autoreactive T cells purified from IL-21-treated mice transferred more severe EAE than did the control encephalitogenic T cells. No such effects were observed when IL-21 was administered after EAE progressed. Additional studies demonstrated that IL-21 given before the induction of EAE boosted NK cell function, including secretion of IFN-γ. Depletion of NK cells abrogated the effect of IL-21. Therefore, IL-21, by affecting NK cells, has differential effects during the initiation and progression of autoimmune responses against neuroantigens.

Tolerogenic Treatment of Lupus Mice with Consensus Peptide Induces Foxp3-Expressing, Apoptosis-Resistant, TGFβ-Secreting CD8+ T Cell Suppressors

Lupus-prone (NZB × NZW)F1 mice spontaneously develop elevated titers of anti-DNA Abs that contain T cell determinants in their VH regions. We have previously shown that tolerization with an artificial peptide based on these T cell determinants (pConsensus (pCons)) can block production of anti-DNA Abs and prolong survival of the mice. In this study, we show that this protection depends in part on the generation of peripheral TGFβ- and Foxp3-expressing inhibitory CD8+ (Ti) cells. These CD8+ Ti cells suppress anti-DNA IgG production both in vitro and in vivo and require up-regulated expression of both Foxp3 and TGFβ to exert their suppressive function, as indicated by microarray analyses, small interfering RNA inhibition studies, and blocking experiments. Additionally, CD8+ Ti cells from pCons-tolerized mice were longer-lived suppressors that up-regulated expression of Bcl-2 and were more resistant to apoptosis than similar cells from naive mice. These data indicate that clinical suppression of autoimmunity after administration of pCons depends in part on the generation of CD8+ Ti cells that suppress secretion of anti-DNA Ig using mechanisms that include Foxp3, TGFβ, and resistance to apoptosis.