A fine romance: IL-7 and HGFβ

Comment on Lai et al, page 1776

In this issue of Blood, Lai and colleagues demonstrate that the recombinant version of the naturally occurring hybrid cytokine IL-7/HGFβ exerts potent expansion of CFU-S, CLPs, and very early B cells, while native IL-7 selectively expands later B-cell progenitor subsets.

Hepatocyte growth factor/scatter factor (HGF/SF) is produced by most all cell types except epithelium. HGF/SF is most well known for its role in liver development and regeneration, but this factor is highly pleiotropic and induces mitogenesis, motogenesis (induction of motility and dissociation, eg, scattering), morphogenesis, and metastogenesis of numerous benign and malignant target cells.¹  A sizable body of evidence also demonstrates that HGF can contribute to expansion of primitive hematopoietic cells, but its exact role in the physiology of hematopoiesis has remained unclear.

Several elements of HGF's biology provide clues to suggest that it may be an important player in the hematopoietic system. First, the receptor for HGF/SF is c-Met, a prototypic receptor tyrosine kinase that is structurally similar to c-kit and FLT3/Flk2, receptor tyrosine kinases with vital roles in hematopoiesis. Second, HGF/SF is produced by marrow stromal cells²  and some hematopoietic cell lines, and c-Met is expressed by hematopoietic progenitor cells and bone marrow stromal cells. Indeed, HGF signaling on marrow stroma has been reported to provide important trophic effects and to induce secretion of IL-11, SDF-1, and SCF.²  Third, HGF/SF is structurally similar to plasminogen, although it has a mutated serine protease domain that prevents it from exerting proteolytic activity. Fourth, functional studies have consistently demonstrated that HGF/SF can induce proliferation of early hematopoietic progenitors,¹  although its effects are consistently augmented by coculture with other cytokines. For instance, HGF/SF synergizes with IL-3 or GM-CSF for expansion of myeloid cell lines and normal hematopoietic progenitors.^3-5 

In 1998, Lai and coworkers identified an IL-7-associated molecular complex that primed differentiating B cells for subsequent proliferation to IL-7 alone (McKenna et al⁶  and Lai et al⁷ ). They named the molecule pre-pro-B cell growth-stimulating factor (PPBSF) and reported that it was a self-assembling heterodimer that could be generated by adding IL-7 to stromal cell-conditioned media from IL-7^-/- mice. Subsequently, this group discovered that the molecule that partnered with IL-7 in PPBSF was a variant beta chain of HGF. Remarkably, this resulted from postsecretion processing, wherein the presence of LMW heparin sulfate-derived oligosaccharides in the microenvironment induced IL-7-HGFβ pairing into a bio-active heterodimer (see figure).⁸ 

Because full characterization of this novel hybrid cytokine could not be performed with the small amounts generated naturally, Lai and colleagues produced a recombinant version of the hybrid cytokine by fusing a signal sequence with IL-7 combined by a linker to HGFβ. The resulting single-chain molecule shows potent and unique activity on primitive hematopoietic cells and developing B-cell subsets. Culture with the hybrid IL-7/HGFβ cytokine results predominantly in expansion of cells bearing a common lymphoid progenitor (CLP) phenotype or a pre-pro-B phenotype, whereas rIL-7 primarily induced expansion of later stage pro-B and pre-B subsets. Furthermore, transfer of marrow cultured with the hybrid cytokine into irradiated hosts generated significantly increased spleen colony-forming units (CFU-S₁₂) compared with results using control marrow or marrow cultured with rIL-7 alone. The numeric advantage of the hybrid's effects persists throughout B-cell development since larger numbers of mature B cells were also present 3 weeks following transfer in mice that received marrows cultured with the hybrid cytokine compared with controls or those receiving marrow cultured with rIL-7 alone. Mechanistically, the data suggest that the hybrid cytokine signals through both the high-affinity IL-7R (IL-7Rα and γc) and c-Met and that the pairing of the chains leads to IL-7R and c-Met aggregation and capping on the surface of stimulated cells.

This work provides a novel insight into yet another mechanism by which cytokines can mediate specificity in vivo. In this case, postsecretion processing leads to selective pairing of genetically unrelated molecules, rendering the resultant signal distinct and greater than the sum of its parts. Whether HGFβ or IL-7 is generally promiscuous and can associate with multiple partners in other settings is yet to be determined. Certainly the previous studies demonstrating synergy between HGF and other cytokines raise the question of whether HGF might also pair with other hematopoietic cytokines to mediate its diverse effects on marrow progenitors. It seems plausible that other cytokines might also use the trick of entering into a low-energy association in specific microenvironments with different partners to mediate their own unique biology. ▪