Mani Mohindru

Cutting Edge: Activation of the p38 Mitogen-Activated Protein Kinase Signaling Pathway Mediates Cytokine-Induced Hemopoietic Suppression in Aplastic Anemia

Myelosuppressive cytokines, in particular IFN-γ and TNF-α, play an important role in the pathogenesis of idiopathic aplastic anemia in humans. It is unknown whether these negative regulators of hemopoiesis suppress stem cells by activating a common signaling cascade or via distinct nonoverlapping pathways. In this study, we provide evidence that a common element in signaling for IFN-γ and TNF-α in human hemopoietic progenitors is the p38/MapKapK-2 signaling cascade. Our studies indicate that pharmacological inhibition of p38 reverses the suppressive effects of IFN-γ and TNF-α on normal human bone marrow-derived erythroid and myeloid progenitors. Most importantly, inhibition of p38 strongly enhances hemopoietic progenitor colony formation from aplastic anemia bone marrows in vitro. Thus, p38 appears to play a critical role in the pathogenesis of aplastic anemia, suggesting that selective pharmacological inhibitors of this kinase may prove useful in the treatment of aplastic anemia and other cytokine-mediated bone marrow failure syndromes.

Differential Virus Replication, Cytokine Production, and Antigen-Presenting Function by Microglia from Susceptible and Resistant Mice Infected with Theiler's Virus

ABSTRACT Infection with Theilers murine encephalomyelitis virus (TMEV) in the central nervous system (CNS) causes an immune system-mediated demyelinating disease similar to human multiple sclerosis in susceptible but not resistant strains of mice. To understand the underlying mechanisms of differential susceptibility, we analyzed viral replication, cytokine production, and costimulatory molecule expression levels in microglia and macrophages in the CNS of virus-infected resistant C57BL/6 (B6) and susceptible SJL/J (SJL) mice. Our results indicated that message levels of TMEV, tumor necrosis factor alpha, beta interferon, and interleukin-6 were consistently higher in microglia from virus-infected SJL mice than in those from B6 mice. However, the levels of costimulatory molecule expression, as well as the ability to stimulate allogeneic T cells, were significantly lower in TMEV-infected SJL mice than in B6 mice. In addition, microglia from uninfected naïve mice displayed differential viral replication, T-cell stimulation, and cytokine production, similar to those of microglia from infected mice. These results strongly suggest that different levels of intrinsic susceptibility to TMEV infection, cytokine production, and T-cell activation ability by microglia contribute to the levels of viral persistence and antiviral T-cell responses in the CNS, which are critical for the differential susceptibility to TMEV-induced demyelinating disease between SJL and B6 mice.

Quantitative, not qualitative, differences in CD8+ T cell responses to Theiler's murine encephalomyelitis virus between resistant C57BL/6 and susceptible SJL/J mice

Theilers murine encephalomyelitis virus (TMEV) infection of the CNS induces an immune‐mediated demyelinating disease in susceptible mouse strains and serves as a relevant infection model for human multiple sclerosis. However, it is not yet clear what immunological parameters determine the susceptibility of SJL/J mice compared to resistant mice. We have here compared the TMEV‐specific CD8+ T cell responses in highly susceptible SJL/J mice with those of highly resistant C57BL/6 mice. Our results clearly indicate that the levels of initial responses of infiltrating CD8+ T cells to viral capsid proteins are higher in resistant C57BL/6 mice compared to susceptible SJL/J mice. However, the level of virus‐specific CD8+ T cells was much more rapidly reduced in resistant C57BL/6, resulting in a higher CD8+ T cell level in SJL/J mice later in viral infection. The activation states, cytokine production, as well as the cytolytic function of the CD8+ T cells were similar to each other in these mice. These results suggest that an initial induction of a vigorous CD8+ T cell response to TMEV is critically important for the resistance to virally induced demyelinating disease.

Pathogenesis of virus-induced immune-mediated demyelination

Theilers murine encephalomyelitis virus-induced demyelinating disease has been extensively studied as an attractive infectiousmodel for human multiple sclerosis. Virus-specific inflammatory Th1 cell responses followed by autoimmune responses to myelin antigens play a crucial role in the pathogenic processes leading to demyelination. Antibody and cytotoxic T cells (CTL) responses to virus appears to be primarily protective from demyelinating disease. Although the role of Th1 and CTL responses in the induction of demyelinating disease is controversial, assessment of cytokine sproduced locally in the central nervous system (CNS) during the course of disease and the effects of altered inflammatory cytokine levels strongly support the importance of Th1 responses in this virus-induced demyelinating disease. Induction of various chemokines and cytokines in different glial and antigen presenting cells upon viral infection appears to be an important initiation mechanism for inflammatory Th1 responses in the CNS. Coupled with the initial inflammatory responses, viral persistence in the CNS may be acritical factor for sustaining inflammatory responses and consequent immune-mediated demyelinating disease.

Initial capsid-specific CD4+ T cell responses protect against Theiler's murine encephalomyelitisvirus-induced demyelinating disease

Central nervous system (CNS) infection by Theilers murine encephalomyelitis virus (TMEV) causes an immune‐mediated demyelinating disease similar to human multiple sclerosis in susceptible mice. To understand the pathogenic mechanisms, we analyzed the level, specificity, and function of CD4+ Th cells in susceptible SJL/J and resistant C57BL/6 mice. Compared to resistant mice, susceptible mice have three‐ to fourfold higher levels of overall CNS‐infiltrating CD4+ T cells during acute infection. CD4+ T cells in the CNS of both strains display various activation markers and produce high levels of IFN‐γ upon stimulation with anti‐CD3 antibody. However, susceptible mice display significantly fewer (tenfold) IFN‐γ‐producing Th1 cells specific for viral capsid epitopes as compared to resistant mice. Furthermore, preimmunization with capsid‐epitope peptides significantly increased capsid‐specific CD4+ T cells in the CNS during the early stages of viral infection and delayed the development of demyelinating disease in SJL/J mice. This suggests a protective role of capsid‐reactive Th cells during early viral infection. Therefore, a low level of the protective Th1 response to viral capsid proteins, in conjunction with Th1 responses to unknown epitopes may delay viral clearance in susceptible mice leading to pathogenesis of demyelination during acute infection, as compared to resistant mice.

Tim-3+ T-bet+ Tumor-Specific Th1 Cells Colocalize with and Inhibit Development and Growth of Murine Neoplasms

Although T cells infiltrate many types of murine and human neoplasms, in many instances tumor-specific cytotoxicity is not observed. Strategies to stimulate CTL-mediated antitumor immunity have included in vitro stimulation and/or genetic engineering of T cells, followed by adoptive transfer into tumor-bearing hosts. In this model of B cell lymphoma in SJL/J mice, we used Tim-3+ T-bet+ Th1 cells to facilitate the development of tumor-specific CTL. Tumor-specific Th1 cell lines were polarized with IL-12 during in vitro stimulation and long term maintenance. As few as 5 million Tim-3+ T-bet+ Th1 cells enabled recipients to resist growth of malignant transplantable cells. In addition, similar numbers of Th1 cells injected into 2- to 3-mo-old mice inhibited development of the spontaneous primary lymphomas, which normally arise in 90% of aging mice. CFSE+ Th1 cells colocalized with injected tumor cells in vivo and formed conjugates with the tumor cells within follicles, whereas in nontumor-challenged recipients the CFSE+ Th1 cells localized only within the T cell zones of the spleen. These results provide evidence that adoptive immunotherapy with Tim-3+ T-bet+ tumor-specific Th1 cells can be used to induce host cytotoxic responses that inhibit the development and growth of neoplastic cells.

Enhanced susceptibility to Theiler's virus-induced demyelinating disease in perforin-deficient mice

Theilers virus induces immune-mediated demyelinating disease similar to human MS in susceptible mice. Though the MHC class II-restricted T cell response is critical, susceptibility/resistance is also associated with a MHC class I haplotype. Here we report that perforin-deficient C57BL/6 mice (pKO) are susceptible to demyelination and develop clinical disease. The levels of primary demyelination, proliferation, Th1 responses, and viral load were also markedly enhanced. In addition, immunization of pKO mice with UV-inactivated virus further enhanced clinical incidence and accelerated the disease course. Thus, perforin is most likely involved in viral clearance, hence protection from the disease.

Clonal Expansion of Infiltrating T Cells in the Spinal Cords of SJL/J Mice Infected with Theiler’s Virus

Intracerebral infection of susceptible mice with Theiler’s murine encephalomyelitis virus results in immune-mediated inflammatory demyelination in the white matter and consequent clinical symptoms. This system has been utilized as an important virus model for human multiple sclerosis. Although the potential involvement of virus-specific Th cells has been studied extensively, very little is known about the nature of T cells infiltrating the CNS during viral infection and their role in the development of demyelinating disease. In this study, the clonal nature of T cells in the spinal cord during the disease course was analyzed using size spectratyping and sequencing of the TCR β-chain CDR3 region. These studies clearly indicate that T cells are clonally expanded in the CNS after viral infection, although the overall TCR repertoire appears to be diverse. The clonal expansion appears to be Ag-driven in that it includes Th cells specific for known viral epitopes. Interestingly, such restricted accumulation of T cells was not detectable in the infiltrates of mice with proteolipid protein peptide-induced experimental autoimmune encephalomyelitis. The initial T cell repertoire (7–9 days postinfection) seems to be more diverse than that observed in the later stage (65 days) of virally induced demyelination, despite the more restricted utilization of Vβ subfamilies. These results strongly suggest continuous stimulation and clonal expansion of virus-specific T cells in the CNS of Theiler’s murine encephalomyelitis virus-infected mice during the entire course of demyelinating disease.

Inhibition of p38α MAPK disrupts the pathological loop of proinflammatory factor production in the myelodysplastic syndrome bone marrow microenvironment

Myelodysplastic syndromes (MDS) are common causes of ineffective hematopoiesis and cytopenias in the elderly. Various myelosuppressive and proinflammatory cytokines have been implicated in the high rates of apoptosis and hematopoietic suppression seen in MDS. We have previously shown that p38 MAPK is overactivated in MDS hematopoietic progenitors, which led to current clinical studies of the selective p38α inhibitor, SCIO-469, in this disease. We now demonstrate that the myelosuppressive cytokines TNFα and IL-1β are secreted by bone marrow (BM) cells in a p38 MAPK-dependent manner. Their secretion is stimulated by paracrine interactions between BM stromal and mononuclear cells and cytokine induction correlates with CD34+ stem cell apoptosis in an inflammation-simulated in vitro bone marrow microenvironment. Treatment with SCIO-469 inhibits TNF secretion in primary MDS bone marrow cells and protects cytogenetically normal progenitors from apoptosis ex vivo. Furthermore, p38 inhibition diminishes the expression of TNFα or IL-1β-induced proinflammatory chemokines in BM stromal cells. These data indicate that p38 inhibition has anti-inflammatory effects on the bone marrow microenvironment that complements its cytoprotective effect on progenitor survival. These findings support clinical investigation of p38α as a potential therapeutic target in MDS and other related diseases characterised by inflammatory bone marrow failure.

Functional maturation of proteolipid protein139–151-specific Th1 cells in the central nervous system in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a widely adopted animal model system for studying human multiple sclerosis that affects the central nervous system (CNS). To understand the underlying pathogenic mechanisms of the autoimmune T cell response, localization, enumeration and characterization of autoreactive T cells are essential. We assessed encephalitogenic proteolipid protein epitope (PLP(139-151))-specific T cells in the periphery and CNS of SJL/J mice using MHC class II I-As multimers during both pre-clinical and clinical phases of PLP-induced EAE in conjunction with T cell function. Our results strongly suggest that PLP(139-151)-specific CD4+ T cells first expand primarily in the CNS-draining cervical lymph nodes and then migrate to the CNS. In the CNS, these PLP-specific CD4+ T cells accumulate, become activated and differentiate into effector cells that produce IFN-gamma in response to the self-peptide.