Long-term Nonprogressive Disease Among Untreated HIV-Infected Individuals: Title and subTitle BreakClinical Implications of Understanding Immune Control of HIV

A 44-year-old black man was diagnosed with human immunodeficiency virus (HIV) infection in 1986. Risk factors included injection drug use and unprotected heterosexual intercourse. He felt well and was lost to follow-up. He was also diagnosed with hepatitis C virus (HCV) infection in 2004. In 2006, his transaminase levels were 4 times the upper limit of normal and his HCV load was 4.7 million IU/mL; however, his CD4 cell count was 1314/μL and his HIV RNA levels were less than 50 copies/mL despite 20 years of untreated HIV infection. Liver biopsy in June 2007 revealed moderate portal inflammation, moderate piecemeal necrosis, and focal mild perisinusoidal fibrosis. A sustained HCV virologic response was achieved with 48 weeks of pegylated interferon alfa-2b and ribavirin combination therapy, completed in June 2008 (Figure 1A). He is antiretroviral naive, yet has maintained normal CD4 counts and HIV RNA levels of less than 50 copies/mL (Figure 1A). Complete blood cell count, serum chemistry, lipid profile, and C-reactive protein results from his last visit in 2009 are within the normal ranges. He carries the HLA antigen HLA class I allele B*57.

A 73-year-old white man reported a rash and high fevers 2 weeks after he had had unprotected intercourse with a male acquaintance in 1987 (when he was 51 years old). He was found to be HIV seropositive. He subsequently learned that his sexual partner's long-term companion had died from AIDS. His sexual partner eventually died from fulminant Pneumocystis jiroveci pneumonia. The patient's medical history included ankylosing spondylitis, ulcerative colitis, type 2 diabetes mellitus, and hypercholesterolemia. Since his initial visit to the National Institutes of Health in 1993, his course has remained stable, with normal CD4 cell counts and suppressed HIV RNA levels, except for an isolated “blip” of 1787 copies/mL in 1998 (Figure 1B). He is antiretroviral naive. Since his last visit in 2009, his complete blood cell count and chemistry profile are within the normal ranges. Serum cholesterol level is well controlled with atorvastatin, and euglycemia is maintained with pioglitazone. He carries the HLA class I allele B*27.

In the era before combination antiretroviral therapy (ART), most HIV-infected individuals experienced progressive decreases in CD4 cell counts during 10 years, culminating in frank immunodeficiency and death from opportunistic diseases.^{1 - 2} However, HIV-induced immune system deterioration did not occur uniformly in all patients. In contrast to the aforementioned typical or “chronic progressors,” some patients, referred to as “rapid progressors,” manifested rapid declines, dying from opportunistic infections shortly after HIV acquisition. At the other end of the spectrum were “long-term nonprogressors” (LTNPs), historically defined as individuals who remained healthy with CD4 cell counts exceeding 500/μL for 10 or more years without ART.¹ With the advent of viral load testing, it became apparent that cohorts of LTNPs classified by this early case definition were heterogeneous with respect to HIV RNA levels. Only a small subset of patients originally classified as LTNPs maintained HIV-1 RNA levels below the lower limits of detection.^{3 - 6} Not surprisingly, these LTNPs with “undetectable” viral loads appeared to be the ones exhibiting truly nonprogressive infection, characterized by stable, nondeclining CD4 cell counts and avoidance of opportunistic diseases during prolonged follow-up. These true LTNPs could be distinguished from “slow progressors” with higher HIV-1 RNA levels (1000-10 000 copies/mL) and discernible CD4 cell losses, and from chronic progressors, who experienced higher viral loads (≥10 000 copies/mL), more accelerated rates of CD4 cell depletion, and the development of opportunistic diseases.^{1 ,3 - 6} Therefore, we have advocated including viral load measurements in addition to stable CD4 cell counts in the case definition to better identify these true LTNPs.⁴ Other investigators have adopted newer designations altogether for true LTNPs, which vary by institution and typically rely only on viral load measurements, without a requirement for stable CD4 cell counts.^{7 - 11} Summary characteristics of these “controller” cohorts are presented in the Table. Quiz Ref IDFor this discussion, we will refer to all patients with “undetectable” viral loads and nonprogressive HIV infection as LTNPs. Regardless of the nomenclature, these unique patients, representing less than 0.5% of the infected population,^{7 ,12} provide evidence supporting the concept that the human immune response can control HIV for many years, which has important implications for the development of effective HIV/AIDS vaccines and immune-based therapies.

The 2 patients described above, with diverse backgrounds but remarkably similar disease courses, are representative of LTNP cohorts being studied at several sites (Table).^{4 ,7 - 11 ,14 - 16} The patients followed at the National Institute of Allergy and Infectious Diseases have experienced positive test results in standard antibody assays, remained healthy without opportunistic diseases, and maintained stable, nondeclining CD4 cell counts and set-point HIV-1 RNA levels of less than 50 copies/mL of plasma without ART for a median of 19 years (Table). Twenty-seven individuals (43%) have untreated infection documented for 20 or more years; 5 (8%) for 25 or more years. A particular sex, racial/ethnic background, or mode of HIV acquisition does not predispose one to become an LTNP, and the composition of these and other demographic/epidemiologic factors varies among different cohorts (Table).

The most important inclusion criterion shared among the various cohorts is maintenance of HIV-1 RNA levels to less than 50 to 75 copies/mL in standard assays (Table). Quiz Ref IDThe degree of this viral load suppression is striking, as demonstrated recently with ultrasensitive assays with a lower detection limit of 1 copy/mL. The median viral load in 2 LTNP cohorts was 2 copies/mL, with one-third possessing HIV RNA levels of less than 1 copy/mL.^{5 ,17} This extremely low-level viremia is comparable to values measured in ART-suppressed patients, although the degree of viral load fluctuation over time might be greater in LTNPs.^{5 ,17 - 19}

Consistent with these impressively low HIV RNA levels, the majority of LTNPs maintain stable, nondeclining CD4 cell counts. However, a small number, especially in cohorts defined solely according to CD4 cell counts, may ultimately experience progressive CD4 cell depletion temporally associated with increasing HIV RNA levels.^{3 ,6} In our experience, only 1 patient has developed accelerated HIV replication, possibly coincident with worsening HCV-related disease, and progressive CD4 cell decline (Figure 2). After a brief course of anti-HCV treatment, he was lost to follow-up for 2 years, until recently presenting with deteriorating health, opportunistic infections, and a CD4 cell count nadir of 122/μL. ART induced marked suppression of HIV replication and some immune reconstitution (Figure 2).

In contrast to the well-known inverse relationship between HIV RNA levels and CD4 cell counts, gradual, progressive CD4 cell decline has also been reported in some LTNPs despite continued suppression of viremia to less than 50 to 75 copies/mL.^{10 ,17 ,20 - 21} In our cohort and others that define LTNPs according to both suppressed viral load and stable CD4 cell counts, progressive disease appears to be rare. However, in cohorts classifying such patients according to viral load or CD4 cell count alone, gradual disease progression may occur more frequently. In one such group, the prevalence of progressive infection, defined as a negative value when the slope of change in absolute CD4 cell counts was calculated over time, was reported to be 10%.¹⁷ Paradoxically, a few “elite controllers” have even met the clinical definition of AIDS, according to CD4 cell counts decreasing below 200/μL or the diagnosis of an opportunistic disease (eg, one case of biopsy-proven cutaneous Kaposi sarcoma at a CD4 cell count of 630/μL).^{10 ,20 - 21} One explanation for CD4 cell loss despite suppressed viremia might be relatively higher HIV-1 RNA levels (≥1 copy/mL) in LTNPs with detectable CD4 cell decline vs lower levels (<1 copy/mL) in individuals with stable CD4 cell counts.¹⁷ However, the majority of patients with HIV-1 RNA levels ranging from greater than or equal to 1 copy/mL to less than 50 copies/mL in our cohort have maintained stable CD4 cell counts for decades, so one cannot predict progressive infection solely according to these sensitive measurements.

Another possible explanation for CD4 cell decline despite suppressed HIV-1 RNA levels is aberrant immune system activation. The generalized inflammatory state induced by HIV infection is thought to be primarily responsible for driving the increased turnover and progressive depletion of CD4 cells observed in most untreated patients.^{22 - 23} Immune system activation is directly associated with viral load but may also be influenced by factors independent of HIV replication. Recent data in LTNPs suggest that disproportionately high T-cell activation might underlie these rare cases of marked CD4 cell decline.^{10 ,20 - 21} As discussed earlier, the utility of such measurements in predicting progressive CD4 cell losses in LTNP remains unclear because, as a group, these patients tend to have more immune activation than HIV-seronegative controls and ART-suppressed patients.^{10 ,24} In the only published case, to our knowledge, of a controller receiving ART because of CD4 cell depletion, reductions in viral load (from 7 to <1 copy/mL) and immune activation occurred after 3 months of therapy. CD4 cell counts stabilized but did not significantly improve during 1-year follow-up.²¹

In summary, the majority of LTNPs do not require antimicrobial prophylaxis or antiretroviral medications to remain healthy for 20 years. However, progressive CD4 cell decline is observed in a small subset, even in those with suppressed viremia. Although the supporting clinical data are sparse, experience in a few isolated cases suggests LTNPs manifesting progressive HIV infection may derive benefit from ART with suppression of HIV RNA levels and stabilization of, or increases in, CD4 cell counts.

The majority of LTNPs exhibit durable control over HIV and are not susceptible to opportunistic diseases. It remains unclear, however, whether LTNP status affects the natural history or response to therapy of HCV or hepatitis B virus in coinfected patients because of limited numbers of cross-sectional studies involving small sample sizes.^{25 - 26} Nonetheless, a few interesting observations deserve mention. Within our cohort, median HCV RNA levels in LTNPs with detectable HCV viremia range from 1.33 million to 6.05 million IU/mL, consistent with HCV/HIV-coinfected patients experiencing progressive HIV disease.^{25 ,27} The rates of spontaneous clearance of both HCV and hepatitis B virus in our coinfected LTNPs are 21.4% (3/14) and 93.9% (31/33), respectively. Although these rates appear to be higher than those of coinfected patients with progressive HIV infection, they do not appear to exceed those of HIV-uninfected persons.^{27 - 28} Larger studies are required to assess clinical outcomes more definitively across patient groups; however, these findings suggest nonimmunodeficient LTNPs are no more likely to control hepatitis B virus or HCV replication than other nonimmunosuppressed individuals.

Quiz Ref IDIn the current era of combination ART, metabolic complications that predispose to premature myocardial infarction, including dyslipidemia, insulin resistance, and overt diabetes mellitus, have been reported in treated HIV-infected patients with increasing frequency.^{29 - 30} Prolonged, uncontrolled HIV infection itself, with its associated immune system activation and possible endothelial dysfunction, appears to be an independent risk factor for metabolic abnormalities and premature cardiovascular disease in untreated patients.^{29 - 30} However, the relevance of these concerns to chronically HIV-infected LTNPs who lack exposure to ART, high-level viremia, and advanced immunodeficiency is unclear.

Some recent data suggest that even the low-level HIV replication and immune activation occurring in LTNPs might be associated with increased risk of cardiovascular complications.¹³ In a cross-sectional study, subclinical atherosclerosis, determined by high-resolution ultrasonographic measurement of carotid artery intima-media thickness, and systemic inflammation, assessed by high-sensitivity C-reactive protein testing, were higher among all HIV-seropositive individuals irrespective of ART or the level of viremia, including LTNPs, compared with uninfected controls.¹³ These differences remained significant even after adjusting for traditional risk factors. Additional data and longitudinal monitoring are needed to assess the clinical significance of these observations in LTNPs and to formulate optimal management strategies. In the meantime, these results at least suggest that aggressive management of traditional cardiac risk factors in all HIV-infected persons may be warranted.¹³

Dramatic declines in HIV-related malignancies have been temporally related to the widespread availability of ART³¹ ; however, cancer rates and disease course in LTNPs remain unknown. In our LTNP cohort, malignancies, which have occurred at a prevalence of 11% (7 of 63), were primarily cutaneous cancers (1 melanoma, 3 basal cell carcinomas, and 2 squamous cell cancers), along with 1 case of Hodgkin lymphoma. As one might expect, none of the classic AIDS-defining malignancies, such as Kaposi sarcoma or non-Hodgkin lymphoma, have been diagnosed among these patients. Only longer-term follow-up will determine whether cancer is likely to become an increasingly important cause of morbidity in nonimmunodeficient LTNPs with prolonged HIV infection.

In summary, although these unique LTNP patients might eventually experience complications related to chronic HIV infection, they have remained clinically well, with extremely low-level viremia for many years. In addition, these individuals provide evidence that control of HIV can be durably maintained by the human immune system.

We recommend that the caregivers of LTNPs follow them regularly, as with other HIV-infected individuals, eg, every 3 to 6 months, and perform routine viral load testing and CD4 cell count determination with standard assays. We also recommend following general guidelines pertaining to health maintenance of all HIV-infected persons, including routine laboratory testing (eg, comprehensive metabolic panel, fasting lipid profile), cancer screening (eg, Papanicolaou smear testing in women), and immunizations (eg, influenza and pneumococcal pneumonia).³² As discussed, a small subset of LTNPs may experience loss of control over viral replication and disease progression. There are currently no assays that can be used to reliably predict progressive infection in LTNPs. In patients who do experience a significant decline in CD4 cell counts or elevation of HIV-1 RNA levels, the antiretroviral treatment and opportunistic infection prophylaxis and treatment guidelines should be consulted and initiation of ART should be considered.^{32 - 33}

Several mechanisms, which have been reviewed in more detail elsewhere,³⁴ have been proposed to explain the extremely low levels of HIV replication observed in LTNPs. These can be broadly divided into virus, host genetic, and host immune response factors. Among virus factors, infection with attenuated or defective viruses that replicate inefficiently has been demonstrated in some rare cases of LTNPs.^{35 - 38} However, several lines of evidence suggest most LTNPs are infected with fully pathogenic, replication-competent virus,^{39 - 40} supporting that host, rather than virus, factors are primarily responsible for reduced HIV replication.

Host genetic polymorphisms that might alter the ability of HIV to gain entry and replicate in host CD4 cells have also been demonstrated in a few LTNPs. For instance, heterozygosity for a 32–base pair deletion in the gene encoding the chemokine receptor CCR5, which mediates the cellular entry of M-tropic strains of HIV, has been associated with delayed disease progression.⁴¹ However, the allelic frequency of this CCR5 Δ32 mutation in our LTNP cohort is 11%, consistent with frequencies of 10% observed in healthy populations of European descent, and is not enriched in other LTNP cohorts.^{4 ,7 ,9 ,37 ,42}

In contrast to the CCR5 Δ32 mutation, the host genetic factor most consistently associated with protection from disease progression is the presence of certain HLA class I B alleles.^{4 ,7 ,43 - 45} These associations with particular HLA class I alleles, which encode for proteins that present HIV peptides to CD8 T cells, are among the strongest described for human diseases. HLA B*5701 is the most significantly overrepresented class I allele observed in most LTNP cohorts (Table) and occurs at a frequency of 65% in our cohort, in contrast to an allelic frequency of 11% in HIV-negative individuals and 10% in cohorts of patients with progressive infection in North America (Table).^{4 ,46 - 47} Similar associations have been observed in several large genome-wide association studies of single-nucleotide polymorphisms in which the factors most strongly associated with delayed disease progression are either HLA class I alleles or single-nucleotide polymorphisms that are in linkage disequilibrium with HLA B*5701.^{48 - 49} Including other HLA class I alleles also reported to be protective, eg, B*13, B*15, B*27, B*44, B*51, and B*58,^{4 ,7 ,43 - 45 ,50 - 51} 90% of the LTNPs in our cohort carry at least one (Table).

Quiz Ref IDThe HLA class I alleles most consistently associated with delayed HIV progression are also strongly associated with some immune-mediated inflammatory conditions: B*57 and Cw*0602, which are in linkage disequilibrium, are enriched in cohorts with psoriasis⁵² and B*2705 is overrepresented in cohorts with the seronegative spondyloarthropathies, including ankylosing spondylitis (eg, patient 2), reactive arthritis, and arthritis associated with psoriasis, Crohn disease, and ulcerative colitis.⁵³ In addition, protective HLA class I alleles have also been associated with drug hypersensitivities, such as the strong association between B*5701 and the hypersensitivity syndrome observed in patients receiving the reverse transcriptase inhibitor abacavir.^{54 - 55} There is evidence supporting a cellular immune basis underlying this abacavir hypersensitivity syndrome.⁵⁵ These findings suggest that a common HLA-mediated mechanism may link these diseases.

These strong associations between the expression of particular HLA class I proteins that present viral peptides to CD8 T cells and LTNP status indirectly support a central role played by HIV-specific CD8 T cells in mediating control over HIV, which is further supported by a wealth of other data. The HIV-specific CD8 T-cell responses of LTNPs with protective HLA alleles preferentially target HIV peptides that bind to the encoded HLA proteins, providing a functional link to the genetic data.^{4 ,56} In addition, some LTNP rhesus macaques have survived for many years with control over simian immunodeficiency virus replication as with LTNP humans, and control in these nonhuman primates has also been associated with a few “protective” major histocompatibility complex class I alleles.⁵⁷ When CD8 cells are depleted from these animals with exogenous antibodies, restriction of simian immunodeficiency virus replication is lost.⁵⁷ Taken together, these data strongly suggest that virus-specific CD8 T cells control lentivirus replication in LTNP macaques and humans.

Quiz Ref IDThe CD8 T cells of LTNPs, compared with those of chronic progressors, do not appear to mediate control over HIV because of larger numbers of HIV-specific CD8 T cells in the peripheral blood or because of a greater ability of these cells to recognize the patients' autologous viruses (reviewed in Migueles et al⁵⁸ ).^{59 - 60} Rather, accumulating evidence points to major differences between LTNPs and patients lacking control over HIV in the functionality of their HIV-specific CD8 T cells.^{5 ,14 ,61 - 63} More specifically, the HIV-specific CD8 T cells of LTNPs compared with the cells of chronic progressors divide robustly when encountering an HIV-infected cell and simultaneously acquire increases in the major proteins contained within cytotoxic granules, the pore-forming protein perforin, and the serine protease granzyme B.^{5 ,64 - 66} By forming pores within target cell membranes, perforin permits granzyme B to gain access to the target cell interior, where it can induce apoptotic cell death (reviewed in Trambas and Griffiths⁶⁷ ). It has been further demonstrated in vitro that increases in these cytotoxic proteins after a period of stimulation confer a superior ability of the HIV-specific CD8 T cells of LTNPs, compared with those of viremic and ART-treated chronic progressors, to kill HIV-infected cells.^{5 ,14} These results demonstrate HIV-specific CD8 T-cell functions that clearly correlate with immunologic control of HIV replication.^{5 ,14}

Long-term nonprogressors serve as important models for effective immunologic control of HIV. They are providing critical information for the development of the T-cell–based component of some HIV vaccines. Understanding the specificities, magnitude, and, most important, the qualitative features of their HIV-specific CD8 T-cell response is providing important clues regarding the goals for the immune response induced by vaccination. Further study of the mechanism of action of particular HLA class I proteins in LTNPs might provide critical insights regarding successful induction and maintenance of an effective antiviral CD8 T-cell response. It remains possible that these mechanisms, if they can be modulated, can be exploited to induce immune-mediated control of HIV.

Unique patients who naturally restrict HIV-1 replication without ART for many years offer hope that durable containment of HIV is possible. Because these LTNPs might be at risk for HIV-related comorbidity and a subset will experience disease progression, routine clinical follow-up and testing are warranted for the entire group. Considerable progress has recently been made in understanding the mechanisms underlying the immune control mediated by HIV-specific CD8 T cells in LTNPs. Further understanding of how these responses arise may have important implications for the development and testing of the next generation of HIV vaccines and immunotherapies.