Normal Th1 development following long-term therapeutic blockade of CD154-CD40 in experimental autoimmune encephalomyelitis

The CD40-CD40L Dyad in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

The CD40-CD40L dyad is an immune checkpoint regulator that promotes both innate and adaptive immune responses and has therefore an essential role in the development of inflammatory diseases, including multiple sclerosis (MS). In MS, CD40 and CD40L are expressed on immune cells present in blood and lymphoid organs, affected resident central nervous system (CNS) cells, and inflammatory cells that have infiltrated the CNS. CD40-CD40L interactions fuel the inflammatory response underlying MS, and both genetic deficiency and antibody-mediated inhibition of the CD40-CD40L dyad reduce disease severity in experimental autoimmune encephalomyelitis (EAE). Both proteins are therefore attractive therapeutic candidates to modulate aberrant inflammatory responses in MS. Here, we discuss the genetic, experimental and clinical studies on the role of CD40 and CD40L interactions in EAE and MS and we explore novel approaches to therapeutically target this dyad to combat neuroinflammatory diseases.

Translational utility of experimental autoimmune encephalomyelitis: recent developments

Multiple sclerosis (MS) is a complex autoimmune condition with firmly established genetic and environmental components. Genome-wide association studies (GWAS) have revealed a large number of genetic polymorphisms in the vicinity of, and within, genes that associate to disease. However, the significance of these single-nucleotide polymorphisms in disease and possible mechanisms of action remain, with a few exceptions, to be established. While the animal model for MS, experimental autoimmune encephalomyelitis (EAE), has been instrumental in understanding immunity in general and mechanisms of MS disease in particular, much of the translational information gathered from the model in terms of treatment development (glatiramer acetate and natalizumab) has been extensively summarized. In this review, we would thus like to cover the work done in EAE from a GWAS perspective, highlighting the research that has addressed the role of different GWAS genes and their pathways in EAE pathogenesis. Understanding the contribution of these pathways to disease might allow for the stratification of disease subphenotypes in patients and in turn open the possibility for new and individualized treatment approaches in the future.

PPE57 induces activation of macrophages and drives Th1-type immune responses through TLR2

Proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) are related proteins exclusive to Mycobacteria that play diverse roles in modulating critical innate immune pathways. In this study, we observed that the PPE57 protein is associated with the cell wall and is exposed on the cell surface. PPE57 enhances Mycobacterium spp. entering into macrophages and plays a role in macrophage phagocytosis. To explore the underlying mechanism, we demonstrated that PPE57 is able to recognise Toll-like receptor 2 (TLR2) and further induce macrophage activation by augmenting the expression of several cell surface molecules (CD40, CD80, CD86 and MHC class II) and pro-inflammatory cytokines (TNF-α, IL-6 and IL-12p40) within macrophages. These molecules are involved in the mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) signalling pathways. We demonstrated that PPE57 effectively polarises T cells to secrete interferon (IFN)-γ and IL-2 and to up-regulate CXCR3 expression in vivo and in vitro, suggesting that this protein may contribute to Th1 polarisation during the immune response. Moreover, recombinant Bacillus Calmette-Guérin (BCG) over-expressing PPE57 could provide better protective efficacy against Mycobacterium tuberculosis challenge compared with BCG. Taken together, our data provides several pieces of evidence that PPE57 may regulate innate and adaptive immunity by interacting with TLR2. These findings indicate that PPE57 protein is a potential antigen for the rational design of an efficient vaccine against M. tuberculosis.

KEY MESSAGES

PPE57 is located on the cell surface and enhances mycobacterium entry into macrophage. PPE57 interacts directly with TLR2 on macrophages. PPE57 plays a key role in the activation of macrophages in a TLR2-dependent manner. PPE57 induces a Th1 immune response via TLR2-mediated macrophage functions. Recombinant BCG over-expressing PPE57 could improve protective efficacy against M. tuberculosis.

CD40L disruption enhances Abeta vaccine-mediated reduction of cerebral amyloidosis while minimizing cerebral amyloid angiopathy and inflammation

Amyloid-beta (Abeta) immunization efficiently reduces amyloid plaque load and memory impairment in transgenic mouse models of Alzheimer's disease (AD). Active Abeta immunization has also yielded favorable results in a subset of AD patients. However, a small percentage of patients developed severe aseptic meningoencephalitis associated with brain inflammation and infiltration of T-cells. We have shown that blocking the CD40-CD40 ligand (L) interaction mitigates Abeta-induced inflammatory responses and enhances Abeta clearance. Here, we utilized genetic and pharmacologic approaches to test whether CD40-CD40L blockade could enhance the efficacy of Abeta(1-42) immunization, while limiting potentially damaging inflammatory responses. We show that genetic or pharmacologic interruption of the CD40-CD40L interaction enhanced Abeta(1-42) immunization efficacy to reduce cerebral amyloidosis in the PSAPP and Tg2576 mouse models of AD. Potentially deleterious pro-inflammatory immune responses, cerebral amyloid angiopathy (CAA) and cerebral microhemorrhage were reduced or absent in these combined approaches. Pharmacologic blockade of CD40L decreased T-cell neurotoxicity to Abeta-producing neurons. Further reduction of cerebral amyloidosis in Abeta-immunized PSAPP mice completely deficient for CD40 occurred in the absence of Abeta immunoglobulin G (IgG) antibodies or efflux of Abeta from brain to blood, but was rather correlated with anti-inflammatory cytokine profiles and reduced plasma soluble CD40L. These results suggest CD40-CD40L blockade promotes anti-inflammatory cellular immune responses, likely resulting in promotion of microglial phagocytic activity and Abeta clearance without generation of neurotoxic Abeta-reactive T-cells. Thus, combined approaches of Abeta immunotherapy and CD40-CD40L blockade may provide for a safer and more effective Abeta vaccine.

Soluble mannosylated myelin peptide inhibits the encephalitogenicity of autoreactive T cells during experimental autoimmune encephalomyelitis

We have previously shown that immunization with a mannosylated myelin peptide in complete adjuvant induces tolerance instead of disease in experimental autoimmune encephalomyelitis (EAE), a rodent model for multiple sclerosis. In this report we demonstrate that treatment with a soluble mannosylated epitope of proteolipid protein (M-PLP(139-151)) significantly inhibits disease mediated by autoreactive myelin-specific T cells during EAE. Treatment with M-PLP(139-151), applied in different EAE models, significantly reduced the incidence of disease and the severity of clinical symptoms. Delayed-type hypersensitivity responses were abolished after peptide treatment, emphasizing the impact on peripheral T-cell reactivity. Histological analysis of spinal cord tissue from mice treated with M-PLP(139-151) revealed the presence of only few macrophages and T cells. Moreover, little expression of interferon-gamma, interleukin-23, or major histocompatibility complex class II antigen was detected. Immune modulation by M-PLP(139-151) was primarily antigen-specific because an irrelevant mannosylated peptide showed no significant effect on delayed-type hypersensitivity responses or on the course of EAE. Therefore, mannosylated antigens may represent a novel therapeutic approach for antigen-specific modulation of autoreactive T cells in vivo.

CD154 blockade results in transient reduction in Theiler's murine encephalomyelitis virus-induced demyelinating disease

Transient CD154 blockade at the onset of Theiler's murine encephalomyelitis virus-induced demyelinating disease ameliorated disease progression for 80 days, reduced immune cell infiltration, and transiently increased viral loads in the central nervous system. Peripheral antiviral and autoimmune T-cell responses were normal, and disease severity returned to control levels by day 120.

Regulation and function of class II major histocompatibility complex, CD40, and B7 expression in macrophages and microglia: Implications in neurological diseases

The ability of microglia, the brain's resident macrophage, to present antigen through the class II major histocompatibility complex (MHC) to T cells allows these normally quiescent cells to play a critical role in shaping the outcome of many neurological diseases. The expression of class II MHC antigens and the costimulatory molecules CD40 and B7 on microglia and infiltrating macrophages is regulated through a complex network of cytokines in the inflamed brain. In this review, we describe the molecular mechanisms underlying class II MHC, CD40 and B7 regulation in microglia and macrophages. Our focus is on the cis-elements in the promoters of their genes and the transcription factors activated by cytokines that bind them. The functional implications of aberrant class II MHC, CD40 and B7 expression by microglia and macrophages as related to the diseases of Multiple Sclerosis and Alzheimer's Disease are discussed.