Herpes simplex virus infection. Epidemiological aspects and analysis of the type-specificantibody response

Abstract

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are ubiquitous viruses, often leading to asymptomatic infections in humans. However, different clinical presentations can be seen including recurrent oral or genital lesions, meningitis and neonatal herpes. HSV-1 and HSV-2 are closely related viruses carrying a genetic homology of 50 %, which has hampered the attempts to differentiate between the two types serologically. Due to the sometimes asymptomatic nature of the HSV-infection, serology can be the only way to diagnose an infection. Type-specific HSV-serology can also be used for epidemiological studies, for counseling in the clinical situation and in evaluating HSV vaccine trials. The envelope glycoproteins G-1 (gG-1) of HSV-1 and gG-2 of HSV-2 are the only known glycoproteins inducing type-specific antibody responses and are now utilized as discriminating ELISA antigens. A prerequisite for type-specific seroassays is, however, that these test are based on genetically stable antigens inducing only type-specific antibodies. In this thesis the B-cell epitopes, including an immunodominant region, have been outlined for gG-1 using the pepscan method. Unexpectedly, the epitopes were mainly localized to regions carrying a high degree of homology to gG-2. Despite this, analysis of purified human anti-gG-1 antibodies displayed no reactivity to the gG-2 antigen. The type-specificity was further explored, after finding two corresponding highly homologous epitopic regions in gG-1 and gG-2, inducing type-specific antibody-responses. Mutational analysis demonstrated that this type-specific reactivity relied on single or dual key residues and was dependant on the structural presentation of these peptides, as shown by molecular modeling. The DNA sequence of gG-1 in clinical isolates was examined, showing that missense mutations affecting the immunodominant region occur. By phylogenetic comparison two different genotypes were established, but the serological response in patients infected with either of these two genotypes did not differ in their IgG reactivity in the gG-1 based ELISA. The type-specific HSV-serology was applied in the clinical setting, using it as a tool for classifying first episodes of genital herpes and clarifying transmission routes. Results revealed that over 60 % of primary genital infections were caused by HSV-1 and that almost 20 % of first episodes of genital herpes were, in fact, the first clinical recurrence of an earlier acquired HSV-2 infection. A suggested explanation to the rise in genital infections caused by HSV-1 has been a decrease of HSV-1 among children. Therefore an epidemiological study was conducted in Swedish children and adolescents, using the type-specific HSV-ELISA:s. In total, HSV-1 IgG antibodies were found in 31 % and HSV-2 IgG antibodies in 0.5 %. The HSV-1 infection seemed to be acquired early in life, with a seroprevalence of over 20 % in the cohort of 1-2 year olds, increasing with higher age. The seroprevalence in the oldest age-cohort did not differ significantly from the HSV-seroprevalence seen in earlier Swedish studies.