Human papillomavirus (HPV) infection is one of the most common sexually transmitted infections and causes cancers in men and women. Over 40 HPV types are known to infect the genital tract, at least 13 of which are considered oncogenic.1 Although infection is most often asymptomatic and transient, persistent infection with oncogenic HPV types is strongly associated with cervical and other anogenital cancers and a subset of head and neck cancers in both men and women.2 Nononcogenic HPV types cause genital warts, other benign lesions, and recurrent respiratory papillomatosis. Serologic assessments measuring antibodies to HPV provide a measure of cumulative exposure to HPV, as antibodies may indicate past infection among those who have no history or current signs of HPV disease. However, at least one important limitation to serologic assessment is that many people with incident or persistent infection do not develop antibodies; in one study, between 54% and 69% of women were found to develop type specific HPV antibodies 18 months after incident HPV infection3; In addition, HPV antibodies may wane over time.3
In the United States, the quadrivalent HPV vaccine is recommended for girls 11 and 12 years of age and girls/women through age 26 years can be vaccinated if they have not received or completed the vaccination series.4 Clinical trials of an HPV vaccine are being completed in men, and results may soon support FDA approval for use in men.5 Although studies have been published on HPV-16 seroprevalence in men in several countries and from different settings including STD clinics, general population, and university clinics,6 few studies have evaluated HPV 6, HPV 11, HPV 16, and/or HPV 18 seroprevalence.7,8,9 Although there are limitations to using seroprevalence for evidence of exposure to vaccine type HPV infection, seroepidemiology in men from various populations can provide important evidence of exposure to these HPV types by age; this information may prove useful in the consideration of HPV vaccine recommendations in men.
MATERIALS AND METHODS
Men aged 18 to 40 years in 2 cities, Tucson, AZ, and Tampa, FL, were enrolled into a cross-sectional study of HPV infection, as described in detail by Nielson et al.10 Men were eligible to enroll if they had had sexual intercourse with a woman within the past year, had no history of genital warts, penile, or anal cancer, and no current symptoms or diagnosis of a sexually transmitted disease. All participants gave informed consent, and all procedures were approved by the University of Arizona Human Subjects Protection Program, Centers for Disease Control and Prevention Institutional Review Board, United States Department of Defense, and the University of South Florida Institutional Review Board.
HPV Serum Antibodies
The study clinician collected 13 mL of venous blood from enrolled men. These specimens were aliquoted and stored at −20°C. HPV antibody status was assessed using techniques previously described. Antibodies against HPV types 6/11 were detected by a combined assay, and HPV 16 and HPV 18 by individual assays performed at Louisiana State University Health Sciences Center. Monoclonal antibodies specific for conformational epitopes on the relevant HPV type were used to detect antibodies by Capture ELISA. The monoclonal antibodies were H11.B10-HPV 6 and 11 (for HPV 6/11), H16.V5-HPV-16 (for HPV 16), and H18.J4-HPV-18 (for HPV 18). Monoclonal antibodies were provided by Dr. N. Christensen (Pennsylvania State Medical Center, Hershey, PA). Participant sera were tested at a 1:100 dilution and tested in triplicate with antigen (HPV capsids and in triplicate without capsids). The log transformation of the median of 3 wells with capsids was subtracted from the median of the 3 wells without capsids to provide the adjusted optical density (aOD). The serological cutpoint was determined from tested negative control sera (children less than 10 years old with no prior history of any warts) and using the average plus 2 standard deviations as the cutpoint.
Use of the negative control sera was approved by Louisiana State University Protocol Detection of antibodies against HPV and by the Institutional Review Board at the Children’s Hospital, where the control-sample blood was collected.
The cutpoint for serum HPV-16 antibody positivity was an aOD of 0.134, for HPV-6/11 antibody positivity was an aOD of 0.367, and for HPV-18 antibody positivity was an aOD of 0.309. All analyses used the cut point to describe seroprevalence. Logistic regression was used to evaluate the associations between demographic and behavioral characteristics of men and the outcome of HPV 6/11, 16, and/or 18 seropositivity. A multivariable model was generated starting with all variables with a P value ≤0.2 in bivariate analysis and then using a backwards elimination approach. All analyses were completed using SAS version 9.0 (Cary, NC).
Of 499 men enrolled in the study who returned for sample collection at the second visit, 498 (99.7%) had results from HPV 6/11, HPV 16, or HPV 18 antibody testing; 462 (92.8%) men had information on age and other covariates and were included in this analysis.
The 462 men were primarily recruited from Tucson, AZ (77.5%), were aged 18 to 24 years (49.8%), predominantly non-Hispanic (82.9%), white (69.9%), and single (70.6%). Most (84.0%) men were circumcised as reported by the provider.
Overall, seroprevalence to HPV 16 was 12.1%, to HPV 6/11 was 9.7%, and to HPV 18 was 5.4%. Ninety-five (20.6%) men were seropositive to HPV 6/11, HPV 16, and/or HPV 18; 13 (2.6%) men were seropositive to both HPV 16 and HPV 18, 3 (0.6%) were seropositive to both HPV 6/11 and HPV 18, and 5 (1%) were seropositive to HPV 6/11 and HPV 16; only 5 (1%) men were seropositive to all types (data not shown).
Seropositivity increased by age group, and was significantly associated with increasing age group for HPV 16 and HPV 18 (Fig. 1). Overall, HPV 16 seropositivity was 5.7% among 18 to 24 year olds and 35.4% among 35 to 40 year olds.
Demographic and behavioral characteristics significantly associated (P < 0.05) with seropositivity to HPV 6/11, HPV 16, and/or HPV 18 in bivariate analysis included age group, race, marital status, history of STD, greater lifetime number of sex partners, greater number of sex partners in the previous 3 months, and current smoking (Table 1). Only 6 men had a wart diagnosis in clinic, and this was not associated with seropositivity to HPV 6/11, HPV 16, and/or HPV 18.
In multivariable analysis, factors independently associated with HPV 6/11, HPV 16, and/or HPV 18 seropositivity were older age (35 to 40 year olds vs. 18 to 25 year olds, adjusted odds ratio (aOR) 6.8 (95% CI, 3.7, 12.8), greater number of female partners in the last 3 months (1 vs. 0, aOR: 4.5 [95% CI, 1.6, 12.6] or >1 vs. 0, aOR: 5.3 [95% CI, 1.8, 15.8]) and current smoking (aOR: 1.9 [(95% CI, 1.1, 3.2]) (Table 2).
In this study of men who had sex with a woman in the past year, 21% were seropositive to HPV 6/11, 16 and/or 18, and seropositivity was highest in the oldest age group (35–40 years). Few men were seropositive to all 4 HPV vaccine-types.
Most studies of HPV seroprevalence in men have reported information on HPV-16. HPV-16 seroprevalence ranges from 3% to 52% in the United States, depending on the population evaluated.6,12–15 Studies in STD clinics have described HPV seroprevalence of 18.7% to 38%,6,14 while studies of the general population have found lower estimates. An assessment in a representative sample of the United States population found that HPV 16 seroprevalence among men was 7.9% (95% CI 6.4, 9.8).12 In a subsequent study using a different serologic assay, HPV 16 seroprevalence was 5.1% (95% CI 4.3, 6.1).9 The seroprevalence in our study (12.1%) was higher than the general population estimates but lower than that found in STD clinics.
Few studies have published information on HPV 6, 11, and 18 seroprevalence in large population based samples,7,8,9 and among men.6,7,9,11–15 Our study found that HPV 6/11 seropositivity was 9.7% and HPV 18 seropositivity was 5.4%. A recently published assessment in a representative sample of the U.S. population evaluated seropositivity to HPV 6, 11, 16 and 18 among male and female participants.9 This study found HPV 6 seropositivity among males was 6.3%, HPV 11 seropositivity was 2.0%, and HPV 18 seropositivity was 1.5%;9 peak seropositivity to HPV 6, 11, 16 or 18 was 18% among 50–59 year olds and varied by HPV type.9 Another study from Australia reported HPV 6 seropositivity among males was 9.1%, HPV 11 seropositivity was 5.2%, and HPV 18 seropositivity was 3.9%. Our study found higher seropositivity compared with the population-based study in the U.S., although it is difficult to compare as these assessments used different serologic methods.
Our study found that current smoking, older age and greater numbers of recent sex partners were independently associated with vaccine-type HPV seropositivity. Sexual behavior and older age have been consistently associated with HPV seroprevalence in men11–16; most studies have evaluated these factors in association with HPV 16 seroprevalence. The finding that recent rather than lifetime sex partners was associated with seropositivity could suggest recent exposure is important for antibody detection in men or that HPV antibody wanes over time. Stone et al reported increasing years of sexual activity and younger age at sexual debut were independently associated with HPV 16 seropositivity.12 Stone et al also reported that older age was a risk factor for HPV 16 seropositivity with peak seroprevalence among men 30 to 39 years of age. Finally, smoking is a well described risk factor for incident and persistent HPV infection, as well as cervical precancers,16,17 but has not been consistently described as a risk factor for HPV seropositivity.
There are several limitations to our findings. Our population included men who were sexually active with at least 1 lifetime sex partner from 2 cities, and who had no current sexually transmitted infection and no history of genital warts; this cohort of men may not represent all men in the general population. Also, this assessment was cross-sectional and could not assess determinants of development of antibodies to HPV 6/11, HPV 16, or HPV 18 after exposure. Finally, our study was restricted to men aged 18 to 40 years, therefore we could not characterize seropositivity in men younger than 18 years or older than 40 years. A limitation in interpretation of all HPV seropositivity data are that seropostivity may underestimate cumulative exposure as antibodies may wane over time, and not all exposures result in seroconversion.
In summary, this study found that many sexually active men, especially with increasing age, had evidence of exposure to at least 1 vaccine-type HPV infection. Few men had antibodies to all 4 vaccine types. This information could be useful when considering HPV vaccine use in men. Recently presented data suggest high efficacy of the HPV vaccine for prevention of persistent infection and external genital lesions in men.5 As in females, a prophylactic HPV vaccine would optimally be given to males before sexual debut, before exposure to HPV is highly probable. Ongoing natural history studies in men, and seroprevalence from a nationally representative survey of men in the United States will also contribute to the considerations of HPV vaccine in men.9
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