Bone marrow B cell apoptosis during in vivo influenza virus infection requires TNF-alpha and lymphotoxin-alpha.

Suppression of bone marrow myeloid and erythroid progenitor cells occurs after infection with a variety of different viruses. In this study, we characterize the alterations in bone marrow (BM) lymphocytes after influenza virus infection in mice. We found a severe loss of BM B cells, particularly CD43(low/-)B220(+) pre-B and immature B cells, in influenza virus-infected mice. Depletion of BM B lineage cells resulted primarily from cell cycle arrest and most likely apoptosis within the BM environment, rather than from increased trafficking of BM emigrants to peripheral lymphoid tissues. Use of gene-knockout mice indicates that depletion of BM B cells is dependent on TNF-alpha, lymphotoxin-alpha, and both TNF receptors, TNFR1-p55 and TNFR2-p75. Thus, TNF-alpha and lymphotoxin-alpha are required for loss of BM B lineage cells during respiratory infection with influenza virus.

Characterization of the in vivo function of TNF-alpha-related apoptosis-inducing ligand, TRAIL/Apo2L, using TRAIL/Apo2L gene-deficient mice.

To define the normal physiological role for the TRAIL/Apo2L in vivo, we generated TRAIL/Apo2L gene-targeted mice. These mice develop normally and show no defects in lymphoid or myeloid cell homeostasis or function. Although TRAIL/Apo2L kills transformed cells in vitro, TRAIL/Apo2L(-/-) mice do not spontaneously develop overt tumors at an early age. However, in the A20 B cell lymphoma-transferred tumor model, TRAIL/Apo2L(-/-) mice are clearly more susceptible to death from overwhelming tumor burden, due to increased lymphoma load in the liver. A20 tumors are susceptible to TRAIL/Apo2L killing in vitro, indicating that TRAIL/Apo2L may act directly to control A20 cells in vivo. Despite the fact that TRAIL binds osteoprotegerin and osteoprotegerin-transgenic mice are osteopetrotic, TRAIL/Apo2L(-/-) mice show no evidence of altered gross bone density, and no alterations in frequency or in vitro differentiation of bone marrow precursor osteoclasts. Moreover, leucine zipper TRAIL has no toxicity when repeatedly administered to osteoprotegerin(-/-) mice. Thus, TRAIL/Apo2L is important in controlling tumors in vivo, but is not an essential regulator of osteoprotegerin-mediated biology, under normal physiological conditions.

RIP4 is an ankyrin repeat-containing kinase essential for keratinocyte differentiation.

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.

A RANK/TRAF6-dependent signal transduction pathway is essential for osteoclast cytoskeletal organization and resorptive function.

Signaling through receptor activator of nuclear factor-kappaB (RANK) is essential for the differentiation and activation of osteoclasts, the cell principally responsible for bone resorption. Animals genetically deficient in RANK or the cognate RANK ligand are profoundly osteopetrotic because of the lack of bone resorption and remodeling. RANK provokes biochemical signaling via the recruitment of intracellular tumor necrosis factor receptor-associated factors (TRAFs) after ligand binding and receptor oligomerization. To understand the RANK-mediated signal transduction mechanism in osteoclastogenesis, we have designed a system to recapitulate osteoclast differentiation and activation in vitro by transfer of the RANK cDNA into hematopoietic precursors genetically deficient in RANK. Gene transfer of RANK constructs that are selectively incapable of binding different TRAF proteins revealed that TRAF pathways downstream of RANK that affect osteoclast differentiation are functionally redundant. In contrast, the interaction of RANK with TRAF6 is absolutely required for the proper formation of cytoskeletal structures and functional resorptive activity of osteoclasts. Moreover, signaling via the interleukin-1 receptor, which also utilizes TRAF6, rescues the osteoclast activation defects observed in the absence of RANK/TRAF6 interactions. These studies are the first to define the functional domains of the RANK cytoplasmic tail that control specific differentiation and activation pathways in osteoclasts.

Cytokine requirements for acute and Basal homeostatic proliferation of naive and memory CD8+ T cells.

Both naive and memory T cells undergo antigen-independent proliferation after transfer into a T cell-depleted environment (acute homeostatic proliferation), whereas only memory T cells slowly divide in a full T cell compartment (basal proliferation). We show, first, that naive and memory CD8+ T cells have different cytokine requirements for acute homeostatic proliferation. Interleukin (IL)-7 receptor(R)alpha-mediated signals were obligatory for proliferation of naive T cells in lymphopenic hosts, whereas IL-15 did not influence their division. Memory T cells, on the other hand, could use either IL-7Ralpha- or IL-15-mediated signals for acute homeostatic proliferation: their proliferation was delayed when either IL-7Ralpha was blocked or IL-15 removed, but only when both signals were absent was proliferation ablated. Second, the cytokine requirements for basal and acute homeostatic proliferation of CD8+ memory T cells differ, as basal division of memory T cells was blocked completely in IL-15-deficient hosts. These data suggest a possible mechanism for the dearth of memory CD8+ T cells in IL-15- and IL-15Ralpha-deficient mice is their impaired basal proliferation. Our results show that naive and memory T lymphocytes differ in their cytokine dependence for acute homeostatic proliferation and that memory T lymphocytes have distinct requirements for proliferation in full versus empty compartments.

Increased susceptibility to tumor initiation and metastasis in TNF-related apoptosis-inducing ligand-deficient mice.

We have previously implicated TNF-related apoptosis-inducing ligand (TRAIL) in innate immune surveillance against tumor development. In this study, we describe the use of TRAIL gene-targeted mice to demonstrate the key role of TRAIL in suppressing tumor initiation and metastasis. Liver and spleen mononuclear cells from TRAIL gene-targeted mice were devoid of TRAIL expression and TRAIL-mediated cytotoxicity. TRAIL gene-targeted mice were more susceptible to experimental and spontaneous tumor metastasis, and the immunotherapeutic value of alpha-galactosylceramide was diminished in TRAIL gene-targeted mice. TRAIL gene-targeted mice were also more sensitive to the chemical carcinogen methylcholanthrene. These results substantiated TRAIL as an important natural effector molecule used in the host defense against transformed cells.