Mucosal calprotectin is associated with severity of aGI-GVHD and poor outcomes after allogeneic stem cell transplantation.

Calprotectin, a calcium- and zinc-binding protein composed of the subunits S100A8 and S100A9, has been extensively studied as a biomarker of gastrointestinal (GI) inflammation through fecal and serum analyses. However, its role in intestinal tissue remains poorly understood due to limited availability of biopsies. In this study, we analyzed S100A8 and S100A9 mRNA expression in 579 intestinal biopsies from allogeneic stem cell transplant (ASCT) patients and observed a strong association with acute GI graft-versus-host disease (aGI-GvHD) (p<0.001). Neutrophil infiltration correlated with the severity of aGI-GvHD (p<0.001), and calprotectin expression was strongly linked to Toll-like receptor 4 (TLR4) (p<0.001) and TLR2 (p<0.001) expression. TLR4 and aGI-GvHD were associated with elevated calprotectin mRNA levels (p<0.001). When patients received broad-spectrum antibiotics at disease onset, expression of calprotectin was suppressed (S100A8, p=0.001; S100A9, p=0.01). Gastrointestinal site-specific differences in calprotectin expression were identified: during severe aGI-GvHD, levels increased up to 30-fold in the small intestine and up to 5-fold in the large intestine with respect to mild/no aGI-GVHD, while under homeostasis, the large intestine exhibited higher baseline calprotectin (p=0.001). The high clinical relevance is evident from the observation that calprotectin expression was prognostic for transplant-related mortality (TRM). Our study suggests that (a) calprotectin is a potential biopsy biomarker in aGI-GvHD, (b) calprotectin expression and neutrophil infiltration possibly indicate translocation of microbiota, which (c) may be modulated by antibiotics.