Advances in Immunology (eBook)

V Growth/Differentiation Factors in Early Thymocyte Development

A GENERAL CONSIDERATIONS

In addition to direct cell-cell contact between thymocytes and thymic stromal cells, soluble factors have long been implicated in the process of thymocyte development. This assumption was primarily based on the fact that thymocytes themselves and thymic stromal cells can produce cytokines (Fischer et al., 1991; Wiles et al., 1992; Wolf and Cohen, 1992; Moore et al., 1993; Keiner et al., 1994; Keiner and Zlotnik, 1995; Mossalayi et al., 1995). In addition, thymocytes express constitutively or inducibly several cytokine receptors in vivo and are responsive to many growth factors in vitro (reviewed by Zlotnik and Moore, 1995). Before the advent of targeted mutants, the potential actions of cytokines and their receptors was analyzed by the addition of exogenous growth factors either to isolated thymocyte subsets in culture or to thymic organ culture systems (see earlier). It was only realized later, much to a surprise, that the vast majority of mutant mice lacking specific cytokines failed to reveal any obvious thymic phenotype as a result of the loss of growth factors or their receptors in the thymus.

B CYTOKINES NOT AFFECTING THYMOCYTE DEVELOPMENT

To date, mice lacking the following cytokines, cytokine receptors, or cytokine-related gene products have been generated with no apparent alteration of thymocyte development: IL-lβ (Zheng et al., 1995; Shornick et al., 1996), IL-1 receptor (IL-1 R) (M. Labow, personal communication), IL-1β converting enzyme (ICE) (Kuida et al., 1995; Li et al., 1995), IL-2 (Schorle et al., 1991), IL-2 R α chain (Willerford et al., 1995), IL-2 R β chain (Suzuki et al., 1997), IL-3 (Nishinakamuraef al., 1996), IL-4 (Kuhn et al., 1991; Kopf et al., 1993). IL-2 and IL-4 (Sadlack et al., 1994), IL-5 (Kopf et al., 1996), IL-8 (Cacalano et al., 1994), IL-10 (Kuhn et al., 1993), IL-12 p40 and p35 (Magram et al., 1996; Mattner et al., 1996), IL-12 R β 1 chain (J. Magram, personal communication), IL-13 (R. Murray, personal communication), tumor necrosis factor (TNF) R 1 (p55) (Pfeffer et al., 1993; Rothe et al., 1993), TNF R 2 (p75) (Erickson et al., 1994), lympho-toxin (De Togni et al., 1994), TNF and lymphotoxin (Eugster et al., 1996), transforming growth factor (TGF) β 1 (Shull et al., 1992; Kulkarni et al., 1993), interferon γ (Dalton et al., 1993; Huang et al., 1993), granulocyte/macrophage (GM) colony-stimulating factor (CSF) (Stanley et al., 1994; Wada et al., 1997), G-CSF R (Liu et al., 1996), the common β subunit for receptors for IL-3, GM-CSF, and IL-5 (Nishinakamura et al., 1995), and fetal liver kinase-2 (flk-2) (Mackarehtschian et al., 1995).

C CYTOKINES AFFECTING THYMOCYTES DEVELOPMENT

In contrast to this long listing of cytokines apparently irrelevant for thymocyte development, lack of very few growth/differentiation factors was shown to affect the generation of thymocytes.

1 IL-6-Related Cytokine Family

Six distinct cytokines sharing structural and functional properties belong to the IL-6-related cytokine subfamily. These are IL-6, IL-11, oncostatin M (OSM), leukemia inhibitory factor (LIF), ciliary neurotropic factor, and cardiotrophin-1 (for further references, see Yoshimura et al., 1996; Taga and Kishimoto, 1997). IL-6 related cytokines bind to cytokine receptors utilizing a common signal transducing receptor subunit termed gpl30 (Gearing et al., 1992; Taga and Kishimoto, 1997). In the case of IL-6, gpl30 forms, together with the IL-6 R α chain (gp80), a functional IL-6 R. LIF binds with high affinity to a receptor complex formed from the LIF R and gpl30 (reviewed by Taga and Kishimoto, 1997). With low affinity, LIF binds to the LIF R in the absence of gpl30. OSM may also bind to LIF R/gp130 or to the OSM R/gpl30 complex, but OSM can also bind to gp130 in the absence of another receptor molecule (Gearing et al., 1992; Taga and Kishimoto, 1997).

Mice lacking IL-6 (Kopf et al., 1994), LIF (Escary et al., 1993), or the gp130 receptor subunit (Κ. Yoshida et al., 1996), as well as transgenic mice expressing in Τ lineage cells LIF (Shen et al., 1994) or OSM (Clegg et al., 1996), have been generated. Interestingly, evidence has been derived from these mutants to suggest that at least two cytokines of the IL-6 family, i.e., OSM and LIF, can be involved, in addition to affecting the HSC compartment, in thymopoiesis. Data regarding the roles of IL-6, OSM, and LIF in T-cell development are summarized next.

a IL-6

Numbers of thymocytes (and peripheral Τ cells) are reduced by 20-40% in IL-6-defïcient mice (Kopf et al., 1994) with normal expression of TCR, CD4, CD8, CD44, and HSA. Thus, IL-6 may provide proliferative signals to thymocytes and mature Τ cells, but the precise stage of action of IL-6 is not known. However, it should be kept in mind that IL-6 was also shown to be involved in survival or maintenance of HSC and/or committed progenitors as revealed by competitive reconstitution assays in vivo (Bernad et al., 1994). Thus, the reduced number of thymocytes in IL-6 deficient mice may reflect a “HSC phenotype” of the IL-6 mutant rather than lack of intrathymic proliferation.

b Leukemia Inhibitory Factor

LIF can affect the development and proliferation of various cell types, including hematopoietic cells (reviewed by Hilton and Gough, 1991). However, expression of LIF is dispensable for mouse development, as shown by the fact that LIF-deficient mice developed normally (Escary et al., 1993). Within the hematopoietic system, however, mice lacking LIF showed clear alterations: Numbers of both CFU-Sday12 as well as committed precursors for erythroid (BFU-E) and myeloid (GM-CFC) lineages were reduced ~10-fold in bone marrow and spleen compared to wild-type mice. In contrast, total cell numbers in bone marrow, spleen, and thymus were roughly normal., indicating that later hematopoietic compartments were “filled up” to compensate for the lower numbers of early progenitors in LIF-deficient mice. In contrast, transplantation of LIF−/− HSC into wild-type mice revealed normal donor-type reconstitution of hematopoietic lineages. Thus, LIF can act as an environmental (stroma)-derived factor to support the maintenance of the HSC compartments in vivo. These experiments give no indication that LIF is involved in early thymocyte development; however, thymocytes derived from LIF−/− or even LIF+/− mice responded at a much reduced level to mitogenic or allogeneic stimulation (Escary et al., 1993).

In addition to pleiotropic alterations caused by LIF overexpression, such as splenomegaly, acute-phase response, and extramedullary hematopoiesis, dramatic effects were recognized in T-cell tissues (thymus and lymph nodes) in mice overexpressing transgenic LIF under the control of a Τ lineage-specific expression cassette (Shen et al., 1994). Remarkably, flow-cytometric analysis revealed an apparent interconversion of phenotypes between lymphocytes in thymus and mesenteric lymph nodes, i.e., CD4+CD8+ thymocytes were absent from the thymus, but CD4+CD8+ lymphocytes were found in the lymph nodes. The thymus contained only lymphocytes with a mature CD4+CD8− and C04−CD8+ single positive phenotype. The thymic architecture was entirely disrupted in LIF transgenic mice, such that epithelial cells apparently formed B-cell follicles but failed to reveal a normal medulla-cortex organization. Collectively, although LIF is dispensable for the generation of thymocytes (Escary et al., 1993), its overexpression in Τ cells can cause an unprecedented conversion of phenotypes between thymus and lymph nodes (Shen et al., 1994).

c Oncostatin M

OSM is expressed in both hematopoietic lineages and stromal cells. OSM can inhibit proliferation or alter the morphology of various tumor cell types. Moreover, OSM can regulate cytokine production and is itself an immediate early gene induced by the JAK-STAT5 signal transduction pathway (for further references see Malik et al., 1995; Yoshimura et al., 1996). Expression of bovine OSM in transgenic mice under the control of the proximal lck promoter, which drives T-cell-specific expression, resulted in a phenotype resembling some findings reported for LIF transgenic mice (see earlier) (Malik et al., 1995). In addition to abnormal bone growth and...