Detection of a single identical cytomegalovirus (CMV) strain in recently seroconverted young women

Abstract

Background: Infection with multiple CMV strains is common in immunocompromised hosts, but its occurrence in normal hosts has not been well-studied.

Methods: We analyzed CMV strains longitudinally in women who acquired CMV while enrolled in a CMV glycoprotein B (gB) vaccine trial. Sequencing of four variable genes was performed in samples collected from seroconversion and up to 34 months thereafter.

Results: 199 cultured isolates from 53 women and 65 original fluids from a subset of 19 women were sequenced. 51 women were infected with one strain each without evidence for genetic drift; only two women shed multiple strains. Genetic variability among strains increased with the number of sequenced genetic loci. Nevertheless, 13 of 53 women proved to be infected with an identical CMV strain based on sequencing at all four variable genes. CMV vaccine did not alter the degree of genetic diversity amongst strains.

Conclusions: Primary CMV infection in healthy women nearly always involves shedding of one strain that remains stable over time. Immunization with CMVgB-1 vaccine strain is not selective against specific strains. Although 75% of women harbored their unique strain, or a strain shared with only one other woman, 25% shared a single common strain, suggesting that this predominant strain with a particular combination of genetic loci is advantageous in this large urban area.

Figure 1. Interval (in months) between first and last sample tested by PCR.

Samples available for PCR sequencing were collected from CMV-infected women at different times following seroconversion.

Figure 2. Alignment of gB subtypes.

Amino acid alignment encompassing the proteolytic cleavage site of gB subtypes detected in the 53 CMV-infected women. Variants with amino acid changes within a subtype are depicted: gB1 (1 and 1**), and gB3 (3 and 3*). The gB1 prototype is Towne, gB2 prototype is AD169, and gB3 prototype is Toledo. The gB1* and the two variants of gB2 are associated with nucleotide changes only. Considering the five major gB subtypes, the two women with multiple strains had gB5/gB1, and gB4/gB1. The subtype distribution among the 51 women who shed one strain was as follows: 15 women shed gB1 (including variants), 9 women - gB2, 10 women - gB3, 4 women - gB4 and 13 women -gB5.

Figure 3. Alignment of UL144 subtypes.

Amino acid alignment of full-length UL144 subtypes A, B, C, A/C. Among the 51 women who shed one viral strain the distribution of UL144 subtypes was: 6 women -UL144A, 7 women - UL144B1, 14 women - UL144B2, 6 women - UL144B3, 10 women - UL144B4, 1 woman - UL144B6, 3 women - UL144A1/C1 and 4 women - UL144C. The women with multiple strains had C/B3, and A1/B1. Please also refer to the phylogenetic tree, figure 4a.

Figure 4. Phylogenetic tree of UL55 and UL144 subtypes.

Left- UL55 DNA, right- UL144 DNA subtypes. Colored in red are -vaccine recipients, and in black- placebo recipients.

Figure 5. alignment of UL146 subtypes.

Amino acid alignment of UL146 subtypes. Subtype definition by Dolan et al (10) appears in parentheses. Several variants in Dolan's subtypes G8, G9, and G11 have been detected in this cohort and are designated E1, E2, G1, G2, H1, and H2 based on common patterns of a small number of nucleotide and amino acid polymorphisms. Among 37 women the distribution of UL146 subtypes was: 14 women - G9, 1- G9*, 5 - G7, 5 - G13, 3 - G11, 2 - G11*, 2 - G8, 1- G8*, 2 - G5, 2 - G1, 1 - G12, and 1- G14. The women with multiple strains had G8*/G11, and G13/G14.

Figure 6. Breakdown of strain diversity in the study cohort based on sequencing of three hypervariable CMV genes.

* The two women with two strains each had one unique strain (based on UL55/UL144) and one shared strain.