Effect of serum and CTL on focal growth of human cytomegalovirus

Background: In immunocompromised patients only cytotoxic T-lymphocytes (CTL) but not antiviral antibodies appear to be efficient in control of human cytomegalovirus (HCMV) infection. This is contrasted by the well-documented neutralising activity of patient sera against standard HCMV strains. Objective: We tested the hypothesis that a cell-culture model based on a recent clinical HCMV isolate would more accurately approximate the clinical situation and provide an explanation for the failure of neutralising antibodies in efficient restriction of HCMV infection. Methods: Sera from five bone marrow transplant recipients with or without prolonged HCMV replication were analysed by an enzyme-linked immunoassay for their capacity to neutralise cell-free HCMV preparations. The inhibitory effect of these sera on viral cell-to-cell-spread was then quantified by focus expansion assays using a recent clinical HCMV-isolate and was finally compared to the inhibitory effect of HCMV-specific CTL lines. Results: Prolonged HCMV replication occurred in three patients despite high titres of neutralising antibodies. In contrast to the strong inhibitory effect on cell-free HCMV, their sera could not inhibit the focal growth of a recent cell-associated HCMV isolate, whereas CTL clones directed against pUL123 or pUL83 of HCMV effectively limited focal expansion of the clinical isolate in fibroblast culture. Conclusions: Focus expansion assays based on a cell-associated clinical HCMV isolate provide a model for the in vivo effectiveness of virus-specific CTL and neutralising antibodies. Our data support the assumption that due to their strict cell-association clinical HCMV strains are withdrawn from neutralising antibodies. (C) 2006 Elsevier B.V. All rights reserved.