Betaherpesvirus Reactivation and Toll-Like Receptor Expression After Allogeneic Stem Cell Transplantation

β-herpesviruses, such as CMV and HHV6, are important pathogen in transplanted patients. The morbidity because of CMV reactivation after allogeneic stem cell transplantation (SCT) has led to the monitoring of this virus and to introduction of preemptive therapy. However, CMV infection is still one of the most challenging complications, because CMV disease may occur as life-threatening pneumonitis, and may increase the risk of opportunistic infections. HHV6 reactivation has been demonstrated after SCT and this virus is recognized as important pathogen, either by direct infection or via interaction with CMV. Innate and adaptive immune response against these viruses involves the activation of Toll-like receptors (TLRs). TLRs belong to type I transmembrane glycoprotein receptor family and recognize pathogen-associated molecular patterns (PAMPs). Viral nucleic acids and viral structural proteins, such as glycoproteins, are considered as PAMPs. Endosomal TLRs (TLR3, 7, 8 and 9) recognize viral nucleic acids and some surface TLRs may be involved in the detection of structural proteins. Some clinical and experimental evidences indicate that CMV and HHV-6 can modulate the immune system and influence the immune reconstitution after SCT. However, the role of TLRs in this complex interplay remains unclear, especially in the setting of allogeneic SCT.

The aim of this study was to evaluate the expression of TLRs on lymphocytes and monocytes in relation to CMV and HHV6 reactivation in the early period after allogeneic SCT.

CMV and HHV6 reactivation was monitored weekly by quantitative real-time PCR until the second month after SCT. The expression of TLRs on lymphocytes and monocytes was analysed by flow cytometry as mean fluorescence intensity at day +30 and in any case before CMV or HHV6 reactivation. Functional data were obtained by ELISA assay after TLRs activation. The cell supernatants were collected and assayed for TNF-alpha, IFN-gamma and MCP-1. Relative induction of these cytokines was calculated in relation with unstimulated controls.

CMV reactivation within 2 months after transplantation was observed in 13 out of 33 patients. CMV pneumonitis was observed in 1 patient. HHV-6 reactivation was detected in 1 patient. Median age was 45 years (range, 22–64) and 21 patients were male. TLRs expression and function did not significantly differ in controls and patients without CMV. Lymphocytes of patients with CMV reactivation showed an increased expression of TLR5 (4,1±2,4 vs 2,0±1,7 p=0,008). TLR8 expression was lower on monocytes with CMV reactivation (0,8±0,9 vs 2,0±1,7 p=0,03). MCP-1 relative induction post-stimulation of TLR1 and 8 was significantly decreased in patients with CMV reactivation (p<0,04).

Surface TLR2 and intracellular TLR3 and 9 are reported to recognize CMV by some authors. In our study, surface TLR5 and intracellular TLR8 seem to be involved in the interaction between CMV and the immune system of transplanted patients. In particular, TLR8 could play a protective role. MCP-1 production upon TLR1 and 8 activation negatively correlates with CMV reactivation. The defective immune system after SCT could explain these results, which could be confirmed by the assessment of a larger number of patients and the analysis of other possible interfering factors.

No relevant conflicts of interest to declare.