Heparan Sulfate and Heparin Enhance ERK Phosphorylation and Mediate preBCR-Dependent Events during B Lymphopoiesis

Abstract

As B lineage cells develop, they interact with cells, proteins, and extracellular matrix components of the surrounding microenvironment. In vitro, one critical checkpoint for developing cells occurs as they lose responsiveness to IL-7. These cells require contact with either stromal cells or other B lineage cells to mature. Our results demonstrate that heparan sulfate and heparin are able to promote this transition when added exogenously to the culture system or when heparan sulfate-bearing cell lines are cocultured with primary B cell progenitors. Addition of heparan sulfate or heparin to LPS-stimulated cultures of primary B cell progenitors resulted in more IgM secreted compared with untreated cultures. Heparan sulfate has been reported to be a ligand for the pre-B cell receptor (preBCR). Extending this observation, we found that treatment of preBCR+ cells with heparan sulfate before anti-μ stimulation leads to increased phosphorylation of ERK1/2. Consequently, preBCR+ cells proliferate more in the presence of IL-7 and heparan sulfate, whereas preBCR− cells are unaffected, suggesting that in these experiments, heparan sulfate is not directly affecting IL-7 activity. Heparin treatment of cultures induces many of the same biological effects as treatment with heparan sulfate, including elevated pERK levels in preBCR+ cells. However, heparin reduces the proliferation of cells expressing only the preBCR (opposed to both the preBCR and BCR) possibly due to internalization of the preBCR. Heparan sulfates are present on stromal cells and B lineage cells present in hemopoietic tissues and may provide stimulation to preB cells testing the signaling capacity of the preBCR.

Disruption of heparan sulfate proteoglycan conformation perturbs B-cell maturation and APRIL-mediated plasma cell survival

“…Pre-BCR and IL-7R-mediated signals are indispensable for B-cell development in BM. 28,29 Notably, several studies have shown that HS can interact with both the pre-BCR 30,31 and with IL-7, 32 suggesting that they may play a role in controlling early B lymphopoiesis. However, the critical HSs in these in vitro studies were either exogenously added or expressed by supporting stromal cells, rather than by the B cells, the Glce-deficient cells in our current model.…”

A Unique Role for the λ5 Nonimmunoglobulin Tail in Early B Lymphocyte Development

“…7). In support of this idea, heparan sulfate and galectin-1 appear to enhance the phosphorylation of signaling molecules downstream of the pre-BCR, such as Ig␣ and ERK (28,51,52). All of these models are not mutually exclusive, and ligand-mediated enhancement of autonomous pre-BCR signals may have evolved to maintain those pre-B cells that express only low amounts of the pre-BCR at their cell surface.…”

The pre-B cell receptor: turning autoreactivity into self-defense

“…These tails are not required for SLC assembly [21,22], but protrude from the pre-BCR at a position where the antigen binding site would be located in the BCR [16,19]; thus the tails are accessible for binding of self-antigens. Indeed, we and others demonstrated that the l5 non-Ig-like tail is an autoreactive entity and can interact with bone marrow stroma cell-associated self-antigens, such as heparan sulfate and galectin-1, both of which enhance pre-BCR signals (Box 3) [17,22,23]. In addition, the l5 tail renders the pre-BCR reactive against itself, thereby leading to pre-BCR self-oligomerization and signal initiation [16,24].…”

“…Therefore, the incomplete nature of the antigen binding site in a pre-BCR makes positive selection by foreign antigens an inappropriate driving force for the proliferation of pre-B cells. Indeed, pre-BCR-mediated proliferation of pre-B cells, in contrast to BCR-mediated proliferation of mature B cells, can occur independently of foreign antigens [10,11,17].…”