Sergio A. Lira

Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain

Interleukin-12 (IL-12) is a heterodimeric molecule composed of p35 and p40 subunits. Analyses in vitro have defined IL-12 as an important factor for the differentiation of naive T cells into T-helper type 1 CD4+ lymphocytes secreting interferon-γ (refs 1, 2). Similarly, numerous studies have concluded that IL-12 is essential for T-cell-dependent immune and inflammatory responses in vivo, primarily through the use of IL-12 p40 gene-targeted mice and neutralizing antibodies against p40. The cytokine IL-23, which comprises the p40 subunit of IL-12 but a different p19 subunit, is produced predominantly by macrophages and dendritic cells, and shows activity on memory T cells. Evidence from studies of IL-23 receptor expression and IL-23 overexpression in transgenic mice suggest, however, that IL-23 may also affect macrophage function directly. Here we show, by using gene-targeted mice lacking only IL-23 and cytokine replacement studies, that the perceived central role for IL-12 in autoimmune inflammation, specifically in the brain, has been misinterpreted and that IL-23, and not IL-12, is the critical factor in this response. In addition, we show that IL-23, unlike IL-12, acts more broadly as an end-stage effector cytokine through direct actions on macrophages.

Mesenchymal and haematopoietic stem cells form a unique bone marrow niche

The cellular constituents forming the haematopoietic stem cell (HSC) niche in the bone marrow are unclear, with studies implicating osteoblasts, endothelial and perivascular cells. Here we demonstrate that mesenchymal stem cells (MSCs), identified using nestin expression, constitute an essential HSC niche component. Nestin+ MSCs contain all the bone-marrow colony-forming-unit fibroblastic activity and can be propagated as non-adherent ‘mesenspheres’ that can self-renew and expand in serial transplantations. Nestin+ MSCs are spatially associated with HSCs and adrenergic nerve fibres, and highly express HSC maintenance genes. These genes, and others triggering osteoblastic differentiation, are selectively downregulated during enforced HSC mobilization or β3 adrenoreceptor activation. Whereas parathormone administration doubles the number of bone marrow nestin+ cells and favours their osteoblastic differentiation, in vivo nestin+ cell depletion rapidly reduces HSC content in the bone marrow. Purified HSCs home near nestin+ MSCs in the bone marrow of lethally irradiated mice, whereas in vivo nestin+ cell depletion significantly reduces bone marrow homing of haematopoietic progenitors. These results uncover an unprecedented partnership between two distinct somatic stem-cell types and are indicative of a unique niche in the bone marrow made of heterotypic stem-cell pairs.

Control of microglial neurotoxicity by the fractalkine receptor

Microglia, the resident inflammatory cells of the CNS, are the only CNS cells that express the fractalkine receptor (CX3CR1). Using three different in vivo models, we show that CX3CR1 deficiency dysregulates microglial responses, resulting in neurotoxicity. Following peripheral lipopolysaccharide injections, Cx3cr1−/− mice showed cell-autonomous microglial neurotoxicity. In a toxic model of Parkinson disease and a transgenic model of amyotrophic lateral sclerosis, Cx3cr1−/− mice showed more extensive neuronal cell loss than Cx3cr1+ littermate controls. Augmenting CX3CR1 signaling may protect against microglial neurotoxicity, whereas CNS penetration by pharmaceutical CX3CR1 antagonists could increase neuronal vulnerability.

Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques

Monocytes participate critically in atherosclerosis. There are 2 major subsets expressing different chemokine receptor patterns: CCR2(+)CX3CR1(+)Ly-6C(hi) and CCR2(-)CX3CR1(++)Ly-6C(lo) monocytes. Both C-C motif chemokine receptor 2 (CCR2) and C-X(3)-C motif chemokine receptor 1 (CX3CR1) are linked to progression of atherosclerotic plaques. Here, we analyzed mouse monocyte subsets in apoE-deficient mice and traced their differentiation and chemokine receptor usage as they accumulated within atherosclerotic plaques. Blood monocyte counts were elevated in apoE(-/-) mice and skewed toward an increased frequency of CCR2(+)Ly-6C(hi) monocytes in apoE(-/-) mice fed a high-fat diet. CCR2(+)Ly-6C(hi) monocytes efficiently accumulated in plaques, whereas CCR2(-)Ly-6C(lo) monocytes entered less frequently but were more prone to developing into plaque cells expressing the dendritic cell-associated marker CD11c, indicating that phagocyte heterogeneity in plaques is linked to distinct types of entering monocytes. CCR2(-) monocytes did not rely on CX3CR1 to enter plaques. Instead, they were partially dependent upon CCR5, which they selectively upregulated in apoE(-/-) mice. By comparison, CCR2(+)Ly-6C(hi) monocytes unexpectedly required CX3CR1 in addition to CCR2 and CCR5 to accumulate within plaques. In many other inflammatory settings, these monocytes utilize CCR2, but not CX3CR1, for trafficking. Thus, antagonizing CX3CR1 may be effective therapeutically in ameliorating CCR2(+) monocyte recruitment to plaques without impairing their CCR2-dependent responses to inflammation overall.

C-C chemokine receptor 6|[ndash]|regulated entry of TH-17 cells into the CNS through the choroid plexus is required for the initiation of EAE

Interleukin 17–producing T helper cells (TH-17 cells) are important in experimental autoimmune encephalomyelitis, but their route of entry into the central nervous system (CNS) and their contribution relative to that of other effector T cells remain to be determined. Here we found that mice lacking CCR6, a chemokine receptor characteristic of TH-17 cells, developed TH-17 responses but were highly resistant to the induction of experimental autoimmune encephalomyelitis. Disease susceptibility was reconstituted by transfer of wild-type T cells that entered into the CNS before disease onset and triggered massive CCR6-independent recruitment of effector T cells across activated parenchymal vessels. The CCR6 ligand CCL20 was constitutively expressed in epithelial cells of choroid plexus in mice and humans. Our results identify distinct molecular requirements and ports of lymphocyte entry into uninflamed versus inflamed CNS and suggest that the CCR6-CCL20 axis in the choroid plexus controls immune surveillance of the CNS.

Multiorgan inflammation and hematopoietic abnormalities in mice with a targeted disruption of RelB, a member of the NF-κB/Rel family

RelB, a member of the NF-kappa B/Rel family of transcription factors, has been implicated in the constitutive expression of kappa B-regulated genes in lymphoid tissues. We have generated mice carrying a germline mutation of the relB gene, resulting in the absence of RelB protein and a dramatic reduction of constitutive kappa B-binding activity in thymus and spleen. Mice homozygous for the disrupted relB locus had phenotypic abnormalities including multifocal, mixed inflammatory cell infiltration in several organs, myeloid hyperplasia, splenomegaly due to extramedullary hematopoiesis, and a reduced population of thymic dendritic cells. RelB-deficient animals also had an impaired cellular immunity, as observed in contact sensitivity experiments. Thus, RelB plays a decisive role in the hematopoietic system, and its absence cannot be functionally compensated by any other member of the NF-kappa B/Rel family.

Origin of the Lamina Propria Dendritic Cell Network

CX(3)CR1(+) and CD103(+) dendritic cells (DCs) in intestinal lamina propria play a key role in mucosal immunity. However, the origin and the developmental pathways that regulate their differentiation in the lamina propria remain unclear. We showed that monocytes gave rise exclusively to CD103(-)CX(3)CR1(+) lamina propria DCs under the control of macrophage-colony-stimulating factor receptor (M-CSFR) and Fms-like thyrosine kinase 3 (Flt3) ligands. In contrast, common DC progenitors (CDP) and pre-DCs, which give rise to lymphoid organ DCs but not to monocytes, differentiated exclusively into CD103(+)CX(3)CR1(-) lamina propria DCs under the control of Flt3 and granulocyte-macrophage-colony-stimulating factor receptor (GM-CSFR) ligands. CD103(+)CX(3)CR1(-) DCs but not CD103(-)CX(3)CR1(+) DCs in the lamina propria constitutively expressed CCR7 and were the first DCs to transport pathogenic Salmonella from the intestinal tract to the mesenteric lymph nodes. Altogether, these results underline the diverse origin of the lamina propria DC network and identify mucosal DCs that arise from pre-DCs as key sentinels of the gut immune system.

Alloantigen-presenting plasmacytoid dendritic cells mediate tolerance to vascularized grafts.

The induction of alloantigen-specific unresponsiveness remains an elusive goal in organ transplantation. Here we identify plasmacytoid dendritic cells (pDCs) as phagocytic antigen-presenting cells essential for tolerance to vascularized cardiac allografts. Tolerizing pDCs acquired alloantigen in the allograft and then moved through the blood to home to peripheral lymph nodes. In the lymph node, alloantigen-presenting pDCs induced the generation of CCR4+CD4+CD25+Foxp3+ regulatory T cells (Treg cells). Depletion of pDCs or prevention of pDC lymph node homing inhibited peripheral Treg cell development and tolerance induction, whereas adoptive transfer of tolerized pDCs induced Treg cell development and prolonged graft survival. Thus, alloantigen-presenting pDCs home to the lymph nodes in tolerogenic conditions, where they mediate alloantigen-specific Treg cell development and allograft tolerance.

The origin and development of nonlymphoid tissue CD103+ DCs

CD103+ dendritic cells (DCs) in nonlymphoid tissues are specialized in the cross-presentation of cell-associated antigens. However, little is known about the mechanisms that regulate the development of these cells. We show that two populations of CD11c+MHCII+ cells separated on the basis of CD103 and CD11b expression coexist in most nonlymphoid tissues with the exception of the lamina propria. CD103+ DCs are related to lymphoid organ CD8+ DCs in that they are derived exclusively from pre-DCs under the control of fms-like tyrosine kinase 3 (Flt3) ligand, inhibitor of DNA protein 2 (Id2), and IFN regulatory protein 8 (IRF8). In contrast, lamina propria CD103+ DCs express CD11b and develop independently of Id2 and IRF8. The other population of CD11c+MHCII+ cells in tissues, which is CD103−CD11b+, is heterogenous and depends on both Flt3 and MCSF-R. Our results reveal that nonlymphoid tissue CD103+ DCs and lymphoid organ CD8+ DCs derive from the same precursor and follow a related differentiation program.

CCR6 mediates dendritic cell localization, lymphocyte homeostasis, and immune responses in mucosal tissue.

Chemokine-directed migration of leukocyte subsets may contribute to the qualitative differences between systemic and mucosal immunity. Here, we demonstrate that in mice lacking the chemokine receptor CCR6, dendritic cells expressing CD11c and CD11b are absent from the subepithelial dome of Peyers patches. These mice also have an impaired humoral immune response to orally administered antigen and to the enteropathic virus rotavirus. In addition, CCR6(-/-) mice have a 2-fold to 15-fold increase in cells of select T lymphocyte populations within the mucosa, including CD4+ and CD8+ alphabeta-TCR T cells. By contrast, systemic immune responses to subcutaneous antigens in CCR6(-/-) mice are normal. These findings demonstrate that CCR6 is a mucosa-specific regulator of humoral immunity and lymphocyte homeostasis in the intestinal mucosa.