Background: SCT is an important treatment modality complicated by a number of immune-mediated toxicities, including aGVHD. In animal SCT models, intestinal translocation of endotoxin (LPS) induces production of TNF-α contributing to development of aGVHD. We tested the hypothesis that in human SCT, myeloablative chemoradiotherapy regimens contribute to the early development of both endotoxemia and a profound deficiency in host defense molecules that normally serve to inhibit endotoxin signaling. In aggregate, these effects may potentiate endotoxin-directed innate immunity and contribute to development of aGVHD and other regimen related toxicities.

Methods: Peripheral blood was obtained from 30 patients (43% < 21 yrs, 60% male) at baseline prior to the start of conditioning, Days 0 (Day of SCT), 7, 14, 21, and 28 post-SCT). A single baseline sample was obtained from each sibling SCT donor. Plasma was analyzed for endotoxin (LPS) and endotoxin-interactive proteins bactericidal/permeability-increasing protein (BPI, a potent endotoxin-neutralizing protein of human neutrophils), the acute phase reactants LPS-binding protein (LBP) and soluble CD14 (sCD14), as well as soluble MD-2 (sMD-2). In addition, plasma concentrations of Regulated upon Activation Normal T Cell Expressed and Secreted (RANTES) and monocyte chemoattractant protein-1 (MCP-1), both chemokines that inhibit Toll-like receptor-4 (TLR4)-mediated TNF and IL-6 production, were determined. Monocyte surface expression of LPS-receptor components membrane CD14 (mCD14) and TLR4 was measured by flow cytometry.

Findings: Nearly 90% of patients had endotoxemia at some time point. Maximal fever incidence and nadir ANC were observed D7 after these myeloablative SCT. Endotoxemia was accompanied by a profound deficiency in plasma BPI concentrations which fell >10-fold from baseline to D7 (p < 0.0001) where it was undetectable in 20/30 patients. RANTES levels were also very reduced at D7 (p < 0.0001). At Day 0, monocyte surface expression of mCD14 fell and TLR4 increased (p < 0.05), a pattern consistent with exposure of monocytes to endotoxin early after conditioning. Subsequently, levels of endotoxin-modulating proteins LBP (D7–14) and sCD14 (D7, 21, and 28) increased (p < 0.05). Low plasma concentrations of the endotoxin-neutralizing protein BPI (baseline) and of RANTES (donor, baseline, and D0) significantly correlated with subsequent risk of aGVHD (p < 0.05).

Interpretation: During myeloablative SCT, endotoxemia occurs in the context of deficient circulating levels of BPI and RANTES. These deficiencies likely contribute to greater endotoxin-TLR4 signalling and resulting production of TNF-α (and other cytokines) and may therefore contribute to consequent aGVHD. Therefore, therapies aimed at enhancing endotoxin-neutralization, such as replenishment of deficient endotoxin antagonists, may be effective approaches to reduce toxicities associated with myeloablative HSCT, including aGVHD. We have therefore initiated a multi-institutional clinical trial to explore this hypothesis by administering recombinant BPI (opebecan, XOMA (US) LLC) to patients undergoing myeloablative SCT.

Author notes

Disclosure:Employment: P. Scannon and H. Leung are employed by XOMA (US) LLC. Consultancy: Ofer Levy has consulted for XOMA (US) LLC. Ownership Interests:; P. Scannon and H. Leung own stocks/options in XOMA (U.S.) LLC. Research Funding: Funded in part by XOMA (US) LLC.