Resistant herpes simplex virus type 1 infection: an emerging concern after allogeneic stem cell transplantation

Resistance of herpesviruses to antiviral drugs: clinical impacts and molecular mechanisms

Nucleoside analogues such as acyclovir and ganciclovir have been the mainstay of therapy for alphaherpesviruses (herpes simplex virus (HSV) and varicella-zoster virus (VZV)) and cytomegalovirus (CMV) infections, respectively. Drug-resistant herpesviruses are found relatively frequently in the clinic, almost exclusively among severely immunocompromised patients receiving prolonged antiviral therapy. For instance, close to 10% of patients with AIDS receiving intravenous ganciclovir for 3 months excrete a drug-resistant CMV isolate in their blood or urine and this percentage increases with cumulative drug exposure. Many studies have reported that at least some of the drug-resistant herpesviruses retain their pathogenicity and can be associated with progressive or relapsing disease. Viral mutations conferring resistance to nucleoside analogues have been found in either the drug activating/phosphorylating genes (HSV or VZV thymidine kinase, CMV UL97 kinase) and/or in conserved regions of the viral DNA polymerase. Currently available second line agents for the treatment of herpesvirus infections--the pyrophosphate analogue foscarnet and the acyclic nucleoside phosphonate derivative cidofovir--also inhibit the viral DNA polymerase but are not dependent on prior viral-specific activation. Hence, viral DNA polymerase mutations may lead to a variety of drug resistance patterns which are not totally predictable at the moment due to insufficient information on specific drug binding sites on the polymerase. Although some CMV and HSV DNA polymerase mutants have been found to replicate less efficiently in cell cultures, further research is needed to correlate viral fitness and clinical outcome.

Resistance in viruses other than HIV

The aim of this brief paper is to summarize the ways in which viruses develop resistance to currently licensed anti-viral drugs and to comment on their clinical relevance. Specific examples will be chosen to emphasize basic principles of the development of resistance and readers are referred elsewhere to a summary of resistance in HIV that follows the same principles.

Development of drug-resistant herpes simplex virus infection after haploidentical hematopoietic progenitor cell transplantation

An unusually high incidence of acyclovir- and foscarnet-resistant herpes simplex virus (HSV) infection was noted after lymphocyte-depleted blood hematopoietic progenitor cell (HPC) transplantation from HLA-haploidentical family donors. Fourteen adults with hematologic malignancies underwent blood HPC transplantation from haploidentical family donors. Pheresis products were stringently depleted of T and B cells by immunomagnetic adsorption, and patients received no immunosuppression after transplantation. HSV reactivation occurred in all 7 evaluable HSV-1- or HSV-2-seropositive patients, at a median of 40 days after transplantation. Susceptibility testing of clinically resistant viral isolates demonstrated acyclovir resistance in all 5 cases tested. Second-line therapy produced only partial responses, and in vitro evidence of foscarnet resistance developed rapidly in all 3 patients treated with foscarnet. Healing of lesions coincided with T-cell recovery. The prolonged immunodeficiency associated with stringent lymphocyte depletion of the graft appears to strongly predispose to emergence of drug-resistant HSV. Furthermore, immune reconstitution is necessary for eradication of infection.

Multiple herpes simplex virus infections with various resistance patterns in a matched unrelated donor transplant recipient

A 45-year-old matched unrelated BMT recipient had sequential mucocutaneous herpes simplex virus (HSV) type 2 infections. Five months after BMT, a penile lesion occurred and was cured using acyclovir, as expected from in vitro susceptibility results. The same lesion recurred 1 month later but worsened with acyclovir. The HSV isolate was resistant to acyclovir (IC(50) = 105 microM), and a nucleotide (G) was added to the thymidine kinase gene leading to a premature stop codon. The lesion improved markedly with foscarnet. During this treatment a second HSV infection occurred on the buttocks 2 weeks after the first one and healed completely with acyclovir. This course correlated with in vitro results of the buttock HSV isolate which was foscarnet-resistant (IC(50) = 300 microg/ml) and acyclovir-sensitive. Surprisingly, no mutation gene of the foscarnet-resistant isolate was detected in the DNA polymerase gene. This case shows that an HSV acyclovir-resistant infection may be followed by an acyclovir-sensitive one. Determination of antiviral susceptibility is needed to monitor the treatment of various HSV infections in immunocompromised BMT recipients.

Infection due to acyclovir resistant herpes simplex virus in patients undergoing allogeneic hematopoietic stem cell transplantation

Over an eight-month period from October 1997 to May 1998, four patients who had received bone marrow transplant (BMT) from unrelated donor presented with severe mucosal cutaneous infections involving acyclovir resistant herpes simplex virus 1 (HSV-1). The four isolates were acyclovir (ACV) resistant, three of which were also foscarnet resistant as determined by the dye uptake method. The sequencing of the thymidine kinase (TK) gene did not permit to establish a relation between mutations and resistance to ACV. Three patients were considered as clinically cured of their HSV infection by replacement of ACV or foscarnet with either valacyclovir (one case) or cidofovir (two cases) but eventually two of them died of graft vs host disease. One patient died of extensive HSV infection despite administration of cidofovir. This study emphasizes the importance of monitoring the herpes virus resistance to antiviral drugs in bone marrow transplant recipients and the usefulness of the evaluation of novel antiviral drug for treatment of infections due to strains of HSV resistant to ACV and foscarnet that occur in about 5% of immunocompromised patients.

Chimeric nectin1-poliovirus receptor molecules identify a nectin1 region functional in herpes simplex virus entry

Human nectin1 (hNectin1), an adhesion molecule belonging to the nectin family of the immunoglobulin superfamily, mediates entry of herpes simplex virus (HSV) into cells. The hNectin1 domain that mediates virus entry into cells and also binds glycoprotein D (gD) has been localized to the first N-terminal V-type domain. The poliovirus receptor (PVR) is a structural homolog to nectins, but it cannot function as an HSV entry receptor. hNectin1-PVR chimeras were constructed to functionally locate the site on hNectin1 involved in HSV entry (HSV entry site). The epitope recognized by monoclonal antibody (MAb) R1.302, which is able to block HSV entry, was also located. The chimeric receptors were designed to preserve the overall structure of the V domain. The HSV entry activity mapped entirely to the hNectin1 portion located between residues 64 and 94 (64-94), likely to encode the C, C', and C" beta-strands and intervening loops. In turn, this site consisted of two portions: one with low-level basal activity for HSV entry (77-94), and one immediately upstream (residues 64 to 76) which greatly enhanced the HSV entry activity of the downstream region. The gD-binding site mapped substantially to the same site, whereas the MAb R1.302 epitope also required a further downstream portion (95-102). The involvement of the 64-76 portion is at difference with previous indirect mapping results that were based on competitive binding studies (C. Krummenacher et al., J. Virol. 74:10863-10872, 2000). The A, A', B, D, E, F, and G beta-strands and intervening loops did not appear to play any role in HSV entry. According to the predicted three-dimensional structure of PVR, the C C' C" site is located peripherally in the V domain and very likely represents an accessible portion at the cell surface.