C57BL/6 mice genetically deficient in interleukin 15 (IL-15−/− mice) were generated by gene targeting. IL-15−/− mice displayed marked reductions in numbers of thymic and peripheral natural killer (NK) T cells, memory phenotype CD8+ T cells, and distinct subpopulations of intestinal intraepithelial lymphocytes (IELs). The reduction but not absence of these populations in IL-15−/− mice likely reflects an important role for IL-15 for expansion and/or survival of these cells. IL-15−/− mice lacked NK cells, as assessed by both immunophenotyping and functional criteria, indicating an obligate role for IL-15 in the development and functional maturation of NK cells. Specific defects associated with IL-15 deficiency were reversed by in vivo administration of exogenous IL-15. Despite their immunological defects, IL-15−/− mice remained healthy when maintained under specific pathogen-free conditions. However, IL-15−/− mice are likely to have compromised host defense responses to various pathogens, as they were unable to mount a protective response to challenge with vaccinia virus. These data reveal critical roles for IL-15 in the development of specific lymphoid lineages. Moreover, the ability to rescue lymphoid defects in IL-15−/− mice by IL-15 administration represents a powerful means by which to further elucidate the biological roles of this cytokine.

Bacterial pneumonia is an increasing complication of HIV infection and inversely correlates with the CD4+ lymphocyte count. Interleukin (IL)-17 is a cytokine produced principally by CD4+ T cells, which induces granulopoiesis via granulocyte colony-stimulating factor (G-CSF) production and induces CXC chemokines. We hypothesized that IL-17 receptor (IL-17R) signaling is critical for G-CSF and CXC chemokine production and lung host defenses. To test this, we used a model of Klebsiella pneumoniae lung infection in mice genetically deficient in IL-17R or in mice overexpressing a soluble IL-17R. IL-17R–deficient mice were exquisitely sensitive to intranasal K. pneumoniae with 100% mortality after 48 h compared with only 40% mortality in controls. IL-17R knockout (KO) mice displayed a significant delay in neutrophil recruitment into the alveolar space, and had greater dissemination of K. pneumoniae compared with control mice. This defect was associated with a significant reduction in steady-state levels of G-CSF and macrophage inflammatory protein (MIP)-2 mRNA and protein in the lung in response to the K. pneumoniae challenge in IL-17R KO mice. Thus, IL-17R signaling is critical for optimal production of G-CSF and MIP-2 and local control of pulmonary K. pneumoniae infection. These data support impaired IL-17R signaling as a potential mechanism by which deficiency of CD4 lymphocytes predisposes to bacterial pneumonia.

The physiological role of the TNF receptor (TNFR) family member, RANK, was investigated by generating RANK-deficient mice. RANK −/− mice were characterized by profound osteopetrosis resulting from an apparent block in osteoclast differentiation. RANK expression was not required for the commitment, differentiation, and functional maturation of macrophages and dendritic cells from their myeloid precursors but provided a necessary and specific signal for the differentiation of myeloid-derived osteoclasts. RANK −/− mice also exhibited a marked deficiency of B cells in the spleen. RANK −/− mice retained mucosal-associated lymphoid tissues including Peyer's patches but completely lacked all other peripheral lymph nodes, highlighting an additional major role for RANK in lymph node formation. These experiments reveal that RANK provides critical signals necessary for lymph node organogenesis and osteoclast differentiation.

SummaryPurified CD4 + lymph node T cells were sorted into two populations on the basis of their expression of CD45RB (CD45RB hi and CD45RB 1~ and injected into congenic severe combined immunodeficient (SCID) mice. After a period of time that was dependent on the number of cells injected, the SCID mice that received CD45RBhi/CD4 + T cells developed a wasting disease that was not seen in SCID mice that received the CD4+/CD45RB 1~ cells or whole lymph node cells. At death, SCID mice that received the CD4 +/CD45RB hi cells had increased spleen and lymph node cellularity compared with normal SCID mice and SCID mice that received the CD4+/CD45RB 1~ T cells. The spleen and lymph node contained CD4 + cells and neither CD8 + nor surface immunoglobulin M-positive cells, plus a population of cells that did not express any of those markers. At necropsy, the SCID mice that received the CD4 +/CD45RB hi cells had significant hyperplasia of the intestinal mucosa with significant lymphoid cell accumulation in the lamina propria. Interestingly, mice that received mixtures of whole lymph node or purified CD4 * cells with CD4 +/CD45RB ~ cells did not develop weight loss, indicating that the unseparated CD4 + population contained cells that were capable of regulating the reactivity of the CD4 +/CD45RB hi cells. utoaggressive immunological reactivity can be driven by CD4 § thymus-derived lymphocytes. For instance, it has been demonstrated that experimental autoimmune encephalomyelitis and diabetes can be induced in normal animals by injecting them with CD4 + T cell clones or lines derived from animals with autoimmune disease of that particular tissue (1-5). Also, T cell reactivity to self-antigens can be demonstrated in normal animals by immunizing them with a closely related antigen in a unique way or by depleting them of a regulatory population (6-8). These data indicate that T cells with specificity for self-antigens exist normally, but that their reactivity is controlled by immunoregulatory mechanisms.It is well appreciated that thymus-derived lymphocytes can be categorized according to the cell surface antigens they express. This has allowed the classification ofdass I or II MHCrecognizing T cells based on their expression of CD8 or CD4 (9). Also, recent data indicate that virgin and memory T cells can be distinguished by their expression of other cell surface markers such as CD44 or CD45 (10-12). The ability to associate T cell function with the expression of a unique array of cell surface determinants is useful in studying the function of these populations in isolation as well as in defined combinations. For instance, Powrie and Mason (13) have separated CD4 + T cells based on their expression of CD45R and injected the resultant subpopulations into congenic, athymic (nude) animals. They found that nude rats injected with congenic CD45Rhi/CD4 + T cells developed wasting disease characterized by inflammatory infiltrates in many organs. Rats injected with unfractionated CD4 + cells (a mixture of CD45R hi and CD45R l~ cells) did n...

The epidermis is a stratified, continually renewing epithelium dependent on a balance among cell proliferation, differentiation, and death for homeostasis. In normal epidermis, a mitotically active basal layer gives rise to terminally differentiating keratinocytes that migrate outward and are ultimately sloughed from the skin surface as enucleated squames. Although many proteins are known to function in maintaining epidermal homeostasis, the molecular coordination of these events is poorly understood. RIP4 is a novel RIP (receptor-interacting protein) family kinase with ankyrin repeats cloned from a keratinocyte cDNA library. RIP4 deficiency in mice results in perinatal lethality associated with abnormal epidermal differentiation. The phenotype of RIP4(-/-) mice in part resembles that of mice lacking IKKalpha, a component of a complex that regulates NF-kappaB. Despite the similar keratinocyte defects in RIP4- and IKKalpha-deficient mice, these kinases function in distinct pathways. RIP4 functions cell autonomously within the keratinocyte lineage. Unlike IKKalpha, RIP4-deficient skin fails to fully differentiate when grafted onto a normal host. Instead, abnormal hair follicle development and epidermal dysplasia, indicative of progression into a more pathologic state, are observed. Thus, RIP4 is a critical component of a novel pathway that controls keratinocyte differentiation.