Introduction
Antiretroviral treatment (ART) has transformed human immunodeficiency virus (HIV)
from a deadly disease to a chronic illness that potentially has little effect on life
expectancy. Modern ART can eliminate viremia and lower the risk of transmission. National
data from the United States demonstrate that 81% of infected individuals receiving
ART are virally suppressed [1]. With treatment available, HIV morbidity and mortality
are not determined by opportunistic infections or AIDS-defining illnesses but rather
by non-AIDS–defining conditions, including cardiovascular disease, liver disease,
kidney disease, malignancies, neurocognitive disorders, and even autoimmune diseases
[2]. To some, autoimmunity coexisting with HIV may be surprising; however, its presence
illustrates how HIV’s immunopathology is more consistent with immune dysfunction than
immune suppression alone.
HIV viral load and the resulting decrease in absolute CD4 T cells have historically
served as biomarkers for HIV’s immune suppression and response to treatment. However,
with successful modern ART and viral suppression, absolute CD4 count and HIV viral
load may not accurately reflect the risks facing patients because immune dysfunction
persists despite normalization of CD4 counts [3]. One explanation is that these markers
fail to truly describe HIV’s overall immune dysfunction contributing to today’s morbidity
and mortality. The CD4/CD8 ratio more accurately describes this overall immune dysfunction
and may be a better biomarker for disease progression, response to treatment, morbidity,
and mortality for the virally suppressed. A greater understanding of the CD4/CD8 ratio
and the impact of its manipulation should be a target for future HIV research.
What is the CD4/CD8 ratio?
CD4 helper/inducer cells and CD8 cytotoxic/suppressor cells are 2 phenotypes of T
lymphocytes, characterized by distinct surface markers and functions that mostly reside
in lymph nodes but also circulate in the blood. The normal CD4/CD8 ratio in healthy
hosts is poorly defined. Ratios between 1.5 and 2.5 are generally considered normal;
however, a wide heterogeneity exists because sex, age, ethnicity, genetics, exposures,
and infections may all impact the ratio [4–7]. Normal ratios can invert through isolated
apoptotic or targeted cell death of circulating CD4 cells, expansion of CD8 cells,
or a combination of both phenomena. A low or inverted CD4/CD8 ratio is an immune risk
phenotype and is associated with altered immune function, immune senescence, and chronic
inflammation in both HIV-infected and uninfected populations [8–11].
The prevalence of an inverted CD4/CD8 ratio increases with age. An inverted ratio
is seen in 8% of 20- to 59-year-olds and in 16% of 60- to 94-year-olds [7]. Women
across all age groups are less likely to have an inverted ratio than their male counterparts
[7]. Age- and hormone-related atrophy of the thymus is theorized to explain the differences
between populations. Hormonal influence on the ratio is supported by a correlation
between low plasma estradiol levels, high circulating CD8, and low CD4/CD8 ratios
in women with premature ovarian failure [12]. Mouse models further highlight the importance
of age and estrogen on the CD4/CD8 ratio because lower ratios are reported in mice
following both natural menopause and ovariectomy [13]. Persistence of the thymus is
associated with better ratio recovery in HIV treatment [14].
Are abnormal CD4/CD8 ratios associated with pathology in the HIV negative population?
In the HIV negative population, a low CD4/CD8 immune risk phenotype reflects immune
senescence, is associated with wide-ranging pathology, and may also predict morbidity
and mortality [7,15–22]. Irreversible disruption of self-immunologic tolerance to
endogenous antigens is a hallmark of autoimmune disease. In this setting of immune
dysfunction, an abnormal CD4/CD8 ratio can emerge. Furthermore, while an abnormal
ratio is not uniformly present in all autoimmune diseases, a decreased CD4/CD8 ratio
is consistently seen in systemic lupus erythematosus [15–17]. A low CD4/CD8 ratio
reflects β-cell destruction and may predict diabetes diagnoses in first-degree relatives
of type 1 diabetic probands [18]. In a population study of solid neoplasms, an inverted
CD4/CD8 ratio is associated with metastatic disease as compared with cancer patients
without metastasis [19]. Moreover, following acute myocardial infarction and cardiopulmonary
resuscitation, a fixed low CD4/CD8 ratio is a poor prognostic sign [20]. Despite these
associations, it is important to acknowledge that the presence of a low CD4/CD8 ratio
is not clearly the cause or the effect of the above pathology. This acknowledgment
is further highlighted by the presence of a low ratio in conditions outside the umbrella
of traditional organic pathology, including an association between low ratios and
pessimists [21].
Conflicting literature exists regarding the use of an inverted CD4/CD8 ratio (<1.0)
as a predictor for mortality in elderly HIV-negative populations. Two longitudinal
cohorts of elderly Swedish individuals demonstrated that an inverted ratio (<1.0)
was associated with frailty and mortality [7,10]. These studies helped define the
immune risk phenotype and raised the possibility of using the CD4/CD8 ratio as a biomarker
to stratify risk in elderly populations. Later cohort studies in Spain and the United
Kingdom found that while a low CD4/CD8 ratio was associated with time to death in
unadjusted analyses, no association between the ratio and morbidity was found in multivariable
analyses [22,23]. Moreover, a recent cross-sectional study of frailty and prospective
cohort study of morbidity in residents of Canadian nursing homes found that greater
percentages of central memory CD8+ T cells were more predictive of increased frailty
than other immune phenotypes, including an inverted CD4/CD8 ratio [24]. Thus, the
CD4/CD8 ratio may not be a marker for morbidity and/or mortality in all populations.
Why should CD4/CD8 ratio be used as a marker in the HIV population?
The natural history of untreated HIV infection has opposing effects on circulating
CD4 and CD8 T lymphocytes. Before HIV lowers CD4 cells, circulating CD8 cells will
typically rise in response to the infection, resulting in a low CD4/CD8 ratio [25].
In the setting of ART, some patients will restore CD4 counts and experience a decline
in CD8 counts, leading to normalization of the ratio. For other individuals, however,
despite suppression of the virus and improvement of CD4 levels, the high levels of
circulating CD8 cells are maintained, and their ratios fail to improve [11,26,27].
A recent cross-sectional study of 334 of these virologically suppressed patients demonstrates
that a lower CD4/CD8 ratio during treatment predicts residual HIV viremia (≥1 copy/ml),
as detected by single-copy assay [28]. Whether this residual viremia is a cause or
effect of a lower CD4/CD8 ratio is unknown, but the association highlights the discordant
immune activation and immune senescence in the virologically suppressed.
Today, more researchers are investigating the ratio’s utility as a biomarker and are
examining the link between the CD4/CD8 ratio and outcome in the HIV-positive population
[29,30]. A low ratio, and not the absolute CD4 count, is the primary factor associated
with a lack of desired response following hepatitis B and yellow fever vaccination
[31,32]. Low ratios have been tied with HIV and the development of neurocognitive
disorders, lung cancer, and chronic obstructive pulmonary disease, while elevations
of activated CD8 cells are linked to myocardial infarction [33–36]. Further evidence
demonstrates an overall increased risk of morbidity and mortality in HIV-positive
individuals who fail to normalize their ratio [11,36]. The ratio is independently
associated with markers of age-associated disease, including carotid intima-media
thickness, arterial stiffness, glomerular filtration rate, and sarcopenia [9]. These
studies demonstrate similar evidence of altered immune function and chronic inflammation
as seen in the noninfected elderly cohorts; however, in the HIV-positive population,
the immune activation and senescence are seen at a much younger age [9]. Although
the link between low ratio and poor outcomes is growing, not all studies agree. A
recent observational cohort of virologically suppressed individuals failed to replicate
evidence that the ratio was prognostic for non-AIDS mortality but found that both
low CD4/CD8 ratio and high CD8 count were associated with excess AIDS mortality in
an HIV population that was otherwise healthy [37].
What alters the CD4/CD8 ratio, and do other pathogens aside from HIV change it?
Untreated HIV infection drives the CD4/CD8 ratio lower. In some populations, initiation
of ART can increase the ratio; however, early and continuous treatment is essential.
If ART begins during primary HIV infection, 90% of patients will achieve normalization
of their CD4/CD8 ratio within 6 years of antiviral therapy, and almost all will normalize
within a decade [27]. Conversely, if ART begins during chronic HIV, then the majority
of patients will fail to normalize their ratio even after 14 years of viral suppression
and restoration of CD4 levels to >500 [27]. Treatment interruptions are also deleterious
to the ratio, so early initiation of ART and continuous adherence to therapy should
be stressed [38].
The optimal treatment regimen for ratio normalization is unknown. Integrase inhibitor,
rather than nonnucleoside reverse-transcriptase inhibitor–or protease inhibitor–based
regimens, is theorized to best improve immune dysfunction. Faster CD4/CD8 ratio normalization
with raltegravir- versus efavirenz-based regimens supports the claim of integrase
inhibitor superiority [39]. The impact of newer integrase inhibitors such as dolutegravir
and elvitegravir is not known.
Cytomegalovirus (CMV) infection has a significant impact on the CD4/CD8 ratio in both
the HIV-positive and -negative populations through the expansion of CMV-specific CD8
cells. This accumulation of CMV-specific CD8 cells lowers CD4/CD8 ratios, leading
to the immune risk phenotype [40]. In HIV-uninfected populations, these clonal expansions
are evident in the elderly; however, in the HIV-infected population, these CMV-specific
CD8 clonal expansions are seen at a younger age [41,42]. Whether other chronic infections
such as tuberculosis, dimorphic fungi, toxoplasmosis, or leishmaniasis also lead to
CD8 expansion with altered ratios is less well studied but likely occurs to a greater
degree in HIV-infected than -uninfected individuals.
CMV coinfection could represent a potential therapeutic target for manipulating the
ratio. In HIV and CMV coinfection, persistent low levels of CMV replication are associated
with lower CD4/CD8 ratios both at diagnosis and while on ART [43]. Moreover, reductions
of activated CD8 T cells are seen in the setting of short-term CMV treatment with
valganciclovir in the coinfected [44]. The impact of long-term simultaneous treatment
of HIV and CMV on immune senescence, the CD4/CD8 ratio, and overall morbidity is not
known. Data on the effect of pathogens and treatment (including immunotherapy) on
T-cell subsets are likely generated by many investigators in the PLOS Pathogens community
but are not often included in final publications.
Could the CD4/CD8 ratio serve as a marker for the HIV reservoir?
Early, effective, and uninterrupted ART improves the CD4/CD8 ratio. Early ART is also
shown to reduce the size of the HIV reservoir [27,44]. Therefore, the use of the CD4/CD8
ratio as a peripheral surrogate of the HIV reservoir is a hypothesis worthy of investigation.
Researchers have linked the CD4/CD8 ratio with integrated levels of HIV–DNA in peripheral
blood cells [27,45]. Similarly, an inverse correlation is demonstrated between CD4/CD8
ratio and the frequency of CD4 T cells carrying HIV–proviral DNA [45]. Furthermore,
lower ratios during ART are also associated with persistently higher HIV–DNA despite
measurable HIV–RNA suppression [46]. While raising the ratio above 1.0 is likely a
prerequisite or an associated phenomenon with reservoir reduction, a high ratio alone
is likely insufficient to eradicate the reservoir, particularly in older patients
with late initiation of treatment. If a sturdier relationship between ratio and reservoir
can be proven, then therapies aimed at reducing the size of the viral reservoir may
use the ratio for assessing their success.
Conclusion
Viral suppression and CD4 response will always remain important treatment goals in
HIV management. Yet if treatment success is defined by these parameters alone, then
we may fail to recognize certain risks encountered by today’s HIV population. Evidence
exists to consider the CD4/CD8 ratio a biomarker for assessing risks facing the modern
aviremic HIV population. Yet the impact of other immune stimuli on the ratio, particularly
non-HIV drugs and copathogens, is often unknown. We plan to highlight this research
as it becomes available at www.GameofTcells.medicine.wisc.edu.
Manipulation of the ratio could serve as a potential target for further HIV therapeutic
interventions, and measurement of the ratio may serve as an adequate surrogate for
the HIV reservoir. More knowledge is needed regarding the impact of specific ART regimens
and the simultaneous treatment of coinfections. Immunotherapy as treatment for oncologic
disorders is increasing, yet surprisingly, any impact of immunotherapy on the ratio
is not routinely reported. Researchers using human and nonhuman animal models should
consider using the CD4/CD8 ratio as a marker in their investigations of HIV and other
chronic conditions that can facilitate translation into clinical practice.