Discussion
We found that alphapapillomavirus and betapapillomavirus types were
detected together significantly less often than expected under
independence of HPV types. Some of this was due to intra-genus
clustering: both alphapapillomavirus and betapapillomavirus types are
more likely to be detected with other types from the same genus. This
clustering is likely due to shared common transmission routes with
closely related types. There were still fewer than expected
co-detections of alphapapillomavirus and betapapillomavirus types than
expected after adjusting for intra-genus clustering, though confidence
intervals could not exclude independence.
A substantial amount of previous research has shown that
alphapapillomaviruses tend to cluster together; individuals who are
positive for one alphapapillomavirus type are at a significantly higher
risk of becoming positive for other alphapapillomavirus
types.13-16 Presumably a large part of this
association reflects shared risk factors across all alphapapillomavirus
types, the most important being a common sexual transmission route as
well as host-level susceptibility to infection. Therefore, it is
unsurprising that we also found betapapillomaviruses tend to cluster
together. This presumably also reflects a shared common transmission
route across all betapapillomavirus types. However, what is much more
surprising is the definite lack of clustering between
alphapapillomaviruses and betapapillomaviruses. While the CIs of many of
the measures of association include the null value expected under
independence (1.0), all previous evidence13-16supports that independence between HPV types is not the expectation and
that the expected clustering of HPVs of the alphapapillomavirus genus is
of the order of OR/HRs of at least 1.5-3.0. Therefore, while the CIs
include ORs and HRs of 1.0, we consider that this is a highly unusual
result. We examine below two potential hypotheses for why
alphapapillomaviruses and betapapillomaviruses may be less likely to be
found together than expected.
The first hypothesis relates to potential biological interactions
between genera. This could arise, for example, if infection with a first
genus induces cross-immunity to subsequent infection with the other
genus, or if both genera occupy the same ecological niche and infection
with one genus leads to competitive exclusion of the other
genus.17 We believe this hypothesis to be implausible
for several reasons. HPV antibodies induced by natural infection tend to
be type-specific,25 so it is unlikely that infection
with types from one HPV genera would generate strong neutralizing
cross-protection against infection with types from another HPV genera.
Alphapapillomaviruses and betapapillomaviruses are also believed to have
distinct tissue tropisms with preferred anatomical sites of
infection.26 While alphapapillomaviruses have evolved
to infect the mucosal epithelium, betapapillomaviruses have evolved to
infect the cutaneous epithelium.2 Nonetheless, both
genera are able to infect epithelial sites outside of their preferred
trophic niche,6,8,27 so it is not impossible they
could have overlapping niches leading to competitive exclusion. If this
were the case, we would expect alphapapillomavirus types to be more
likely to exclude cervicovaginal infections with betapapillomavirus
types than the reverse due to their mucosal tropism. However, we did not
find that alphapapillomavirus positivity reduced incident
betapapillomavirus more than the reverse, so the hypothesis of
competitive exclusion does not seem to be supported.
The second hypothesis is that the negative associations between genital
alphapapillomaviruses and betapapillomaviruses could reflect independent
or inversely correlated transmission mechanisms. Alphapapillomaviruses
are sexually transmitted infections, with major risk factors including
lifetime number of sex partners and new sex
partners.28 Betapapillomaviruses are believed to be
transmitted through skin-to-skin contacts,2 which
include but are not restricted to sexual contacts. Interestingly, we
found that lifetime and new sex partners were not risk factors for
betapapillomavirus incidence, but having any sex during the previous
interval was a risk factor. Most of the women who reported having sex in
our study were having sex with a regular partner rather than a new one,
and women who were married or living with their partner reported higher
frequencies of sexual contacts than single or separated women. It could
be that, in this study, having sex is an indicator for being in an
ongoing intimate relationship with more skin-to-skin contacts, which
include but may not be restricted to sex. It is also possible that
betapapillomavirus detections may be depositions from recent sex rather
than true infections, as the penile epithelium has been found to have a
high betapapillomavirus prevalence.9 However, a
previous study did not find that vaginal sex within the last 24 hours
increased genital betapapillomavirus concordance between heterosexual
partners.8 Nevertheless, that study and others have
found that concordance was higher within than between couples,
suggesting some form of transmission through intimate
contact.8,29 Fingernails could be a potential
reservoir for transmission and autoinoculation in the case of oral
betapapillomavirus infections.11 We believe it is
plausible that the negative association observed between
betapapillomavirus and alphapapillomavirus types could partly reflect
confounding due to inversely correlated transmission mechanisms, as
women in our study who were having more regular sex and intimate
skin-to-skin contact were women with fewer lifetime sexual partners or
new sex partners. However, our adjustment for these sexual behaviors in
multivariable models did not completely attenuate the negative
association between alphapapillomaviruses and betapapillomaviruses.
There could therefore remain some unmeasured confounding transmission
risk factors explaining the negative association between
betapapillomaviruses and alphapapillomaviruses.
The most detected betapapillomavirus types in cervicovaginal samples
were HPV38, HPV5, HPV21, HPV22, and HPV8, respectively. Moscickiet al . also found that HPV38 was the most commonly detected
betapapillomavirus type in cervicovaginal samples.8HPV38 is also one of the most commonly detected types in women’s oral
samples,11 and among the most common
betapapillomavirus types in men’s genital samples.9Genital HPV21 and HPV22 positivity were also common in male genital
samples.9 HPV5 and HPV8 were conversely not among the
most commonly reported detected types in male genital
samples.9 HPV5 and HPV8 are the betapapillomavirus
types which have been isolated from patients withepidermodysplasia verruciformis (EV) and are deemed to be
possibly carcinogenic for individuals with EV by the International
Agency for Research on Cancer.1,30 Oral HPV5 has also
been associated with the risk of some head and neck
cancers.3
By increasing the numbers of samples tested for betapapillomavirus, we
were able to expand on our previous results which only included two
observations per woman.19 The current study confirms
our initial surprising findings that lifetime number of sex partners was
inversely correlated with betapapillomavirus prevalence, and we propose
above a potential explanation for this finding based on a multivariable
analysis of sexual risk factors. However, this finding might reflect
confounding from sexual behaviors that is specific to our study
population, as studies of oral and penile betapapillomavirus infections
have not found that number of lifetime sex partners was associated with
betapapillomavirus prevalence at these
sites.9,11,12,31 We are unaware of any other studies
that have looked at risk factors for cervicovaginal betapapillomavirus
prevalence. Due to the larger sample size, we were able to calculate
incidence rates of betapapillomavirus positivity, and also detect lower
than expected prevalent and incident co-detections of
alphapapillomavirus and betapapillomavirus types which were not
previously evident with fewer observations. However, despite the larger
sample size, there were still large CIs for some associations; we
therefore cannot exclude that our results could be due to chance. Our
results would need to be confirmed in other studies of
alphapapillomavirus and betapapillomavirus co-detection to verify
whether they represent a more general trend across populations.
In conclusion, we found fewer prevalent and incident co-detections of
alphapapillomaviruses and betapapillomaviruses than expected if HPV
types belonging to these genera were transmitted independently. It is
not clear whether these highly unusual findings reflect potential
biological interactions between HPV genera, or are the result of
inversely correlated transmission mechanisms, or simply represent a
chance result. These findings need to be confirmed in other populations.
This would help further elucidate the biology of betapapillomaviruses, a
genus for which there is growing evidence of a role in carcinogenesis.