SCFcKit and Lymphopoiesis

7.6.1. T Lymphopoiesis

During mouse embryogenesis, numbers of c-kit+ cells in fetal liver and in fetal thymus reach a maximum at about d 15 and then decline; stromal/epithelial cells in the fetal thymus synthesize SCF mRNA (89). Thymic defects have been noted in Sl and W mutant animals (Subheading 7.1.) In c-kit-/yc- mice, T-lymphoid hypocellularity in the thymus is extreme, and T-cell receptor (TCR)-P chain rearrangement repertoire formation is essentially absent; this severe phenotype probably reflects a normal requirement for cooperation between c-kit signaling and IL-7 receptor signaling (48). In addition, wild-type fetal liver cells can reconstitute T lymphopoiesis when transferred to immun-odeficient mice (RAG-2- or RAG-2-/yc-), but W/W-mutant fetal liver cells do so only partially (90,91). Taken together, these observations suggest requirements for c-kit expression in progenitor cells and for SCF expression in the microenvironment during the very early stages of thymocyte maturation in the mouse thymus. The effects of blocking Ab against c-kit, administered in in vitro thymus organ cultures, support these conclusions and suggest a role for c-kit/SCF in the transition and commitment of T-cell progenitors from bone marrow to thymus (92,93).

Progression in T-cell maturation from very primitive triple-negative (CD4-CD8-CD3-) progenitors to pro-T cells, to pre-T cells, to subsequent terminal differentiation with expression of CD4 and/or CD8 has been delineated (TCR gene rearrangements occur at about the pro-T cell to pre-T-cell transition). c-Kit is expressed up until the terminal differentiation stages (5,7,48,89,92,94,95). SCF is contiguously expressed by thymic stromal cells (human) (94,96).

Several studies of the effects of SCF on immature thymocytes and on T cells in vitro have been reported. SCF in combination with cytokines such as IL-2, IL-3, and IL-7 or with thymic stromal cells supported the growth of adult very primitive thymocytes and pro-T thymocytes (mouse [97] and human [94,98]) but not more mature thymocytes or T cells (mouse; [92,95]).

In vivo administration of SCF to normal mice (99) or baboons (100) over periods of several weeks leads to observable but modest (two- to fourfold) increases in the numbers of circulating lymphocytes. The numbers tend to decrease toward baseline before the end of dosing.

7.6.2. Intraepithelial Lymphocytes

A particular class of adult mouse T cells, the CD8-TCR-y5 IELs found in the intestine, seem to be dependent on SCF/c-kit in adult mice. These cells mature and undergo recombinational TCR gene rearrangements independently of the thymus; are juxtaposed with intestinal epithelial cells in vivo; and are thought to contribute to host firstline immune protection against bacterial and viral infection in the intestinal mucosa. TCR-y5 IEL populations become greatly depleted in W/W and Sl/Sld mice, both in the small intestine and in the large intestine, in an age-dependent fashion that starts approx 6 wk after birth (47,101). In parallel, the TCR-aP IEL population proliferates extensively and undergoes alterations with respect to CD4/CD8 subset ratios, in the small intestine of W/W mice (47,101). Thus IEL homeostasis is disrupted, particularly in the small intestine. c-Kit mRNA and cell surface c-kit protein are expressed by TCR-yS IELs from normal mice (47,101,102), c-kit protein is expressed by potential IEL precursor cells in the intestinal epithelium (103) and is variably expressed by TCR-aP IEL from normal mice (47,102), and SCF mRNA can be expressed by intestinal epithelial cells (101). In vitro, SCF selectively induces proliferation of TCR-yS IELs but not TCR-aP IELs and enhances the activation (IFN-y production and cytolytic activity) of IELs by IL-2 and anti-TCR-yƓ Ab (102,104,105). In vivo administration of SCF in mice can expand the IEL populations (102).

7.6.3. B Lymphopoiesis

SCF/c-kit appears to play stimulatory but not obligatory roles in B lymphopoiesis, more so in the mouse system than in the human. W/W mutant mice are not at all deficient with respect to B-cell maturation during embryogenesis (90). Fetal liver progenitor populations from W/W mutant mice vs wild-type mice are equally proficient in reconstituting B lymphopoiesis when transplanted to immunodeficient recipients (89-91). In vivo administration of blocking Ab against c-kit to adult mice abrogates all hematopoietic lineages except the B-cell lineage, which is actually enhanced and expanded, possibly because progenitors are blocked from progressing along other lineage maturation paths (53). Thus, of the hematopoietic cell lineages, B lymphopoiesis is probably the least affected by SCF/c-kit; SCF/c-kit can be stimulatory but clearly are not obligatory.

In the mouse, c-kit is heterogeneously expressed in the earlier stages of B-cell maturation, i.e., pro-B (TdT+, B220-) and pre-B (B220+, c|+, slg-) (89). Loss of c-kit expression seems to coincide with maturation (including Ig rearrangement in the pre-B stage) and lineage restriction (53,106), and mature B cells are c-kit- (89). In culture, SCF, supplied exogenously or produced by cocultured stromal cells, can enhance the recruitment of pro-B cells from a primitive Sca-1+c-kit+Flt3-Lin-/low population when present along with IL-7 and Flt3 ligand (107) and can greatly expand the output of maturing B cells when present along with IL-7 (107-109), but its absence does not limit the progression of B-cell maturation.

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