Increases in CD4 cell counts seen with growth hormone, interleukin-7

Significant increases in CD4 cell counts have been seen in HIV-positive individuals treated with interleukin-7 and with growth hormone. Growth hormone has also been shown to stimulate naive CD4 cell proliferation and growth of the thymus – the gland in which T cells mature. The studies were presented to the recent Conference on Retroviruses and Opportunistic Infections and published in the March edition of the Journal of Clinical Investigation.

Interleukins are a subtype of the molecules known as cytokines – molecular messengers which regulate the function of the immune system. Several interleukins, particularly interleukin-2 (IL-2), play a role in promoting CD4 cell production, and considerable research has explored the potential of interleukins for boosting CD4 cell counts.

While most studies have focused on IL-2, the relatively little-studied interleukin-7 (IL-7) is also involved in the proliferation of T-cells. Animal studies have shown that IL-7 can enhance T-cell proliferation, and low IL-7 levels have been linked to the relatively poor CD4 increases seen with antiretroviral therapy in HIV/hepatitis C co-infected individuals.

In a phase I/II study presented as a poster at the Fifteenth Conference on Retroviruses and Opportunistic Infections (CROI), a French research team investigated the 48-week safety and preliminary efficacy of a recombinant version of IL-7 (r-hIL-7, manufactured by Cytheris SA) in 13 chronically HIV-infected patients. (Twelve-week results were previously reported at last year's CROI.) Participants were required to be virologically suppressed (plasma HIV RNA below 50 copies/ml) on antiretroviral therapy for at least six months, and to have CD4 cell counts between 100 - 400 cells/mm³. In the initial phase of the study, participants received eight subcutaneous injections of r-hIL-7, given three times per week during study days 1 to 16. Six patients received a 3 μg/kg dose and seven received a 10 μg/kg dose at each injection. Long-term follow-up then continued for 48 weeks.

In the 3 μg/kg group, HIV viral load remained suppressed below 50 copies/ml in all six patients throughout the 48-week follow-up. Transient viral load increases were seen in three of the seven patients in the 10 μg/kg group at weeks 12 (630, 74, and 120 copies/ml) and 24 (500, 130, and 200 copies/ml), and one of three patients at week 48 (840 copies/ml). According to the investigators, "no meaningful clinical events" were noted during follow-up. Mild to moderate localised injection site reactions and transient flu-like symptoms were common. There was one transient grade 3 elevation of liver enzymes in a patient with hepatic steatosis (ALTs above 10x ULN); this resolved spontaneously and no other adverse events were reported during the study period.

In the 3 μg/kg group, baseline CD4 cell counts were 216 cells/mm³; they increased by 68% to 369 cells/mm³ at day 28 and remained at 282 cells/mm³ at week 48, a 28% increase over baseline (p = 0.0001). CD8 cells increased by 21% from a baseline of 848 cells/mm³ to 649 cells/mm³ at week 48 (p = 0.01).

In the 10 μg/kg group, baseline CD4 cell counts were 239 cells/mm³. At 48 weeks they had increased by a median of 75% to 403 cells/mm³ (p = 0.0001); CD8 cell counts rose 57% from a baseline of 580 cells/mm³ to 1011 cells/mm³ at week 48(p = 0.0001).

Although this was a preliminary study in a small number of patients, the investigators observed that "r-hIL-7 administration significantly improves CD4 T cell counts in patients with low CD4 counts," with significant increases in HIV-specific CD4 responses seen at week 12. Immunological analyses up to week 12 showed that CD4 cell increases occurred predominantly in memory cells, while CD4 cells showed increases in both memory and naive cell populations.

The clinical significance of these findings is unknown. It should be noted, for instance, that the dramatic increases in CD4 cell counts seen in many patients in IL-2 studies have not necessarily been tied to equally dramatic improvements in clinical outcome. There was also some concern about the observed "blips" in viral load, noting that the "possible effect [of IL-7] on HIV replication should be carefully monitored in future studies." However, as the investigators conclude, their results warrant further investigation and "reinforce interest in the clinical use of IL-7 in the treatment of HIV infection."

Meanwhile, another small but in-depth human trial has demonstrated that human growth hormone treatment increases overall CD4 cell count, the number of naive CD4 cells, the degree of T cell activation, and the mass and functionality of the thymic gland in HIV-positive individuals on antiretroviral therapy.

Growth hormone, a substance naturally produced by the body, promotes bodily growth and development. An artificially-produced version of the hormone, called recombinant human growth hormone (rhGH), has been used to combat AIDS-related wasting and lipodystrophy. Preliminary studies have shown that growth hormone can also stimulate the production of CD4 cells in patients with a poor immunologic response to antiretroviral treatment, as well as increasing the mass and functionality of the thymus, or thymic gland (the site of T-cell maturation, previously thought to irreversibly deteriorate with age and HIV infection).

Early results of one such study, by a team at the University of California San Francisco and the Gladstone Institute of Virology and Immunology, were reported at the Third International AIDS Society Conference in 2005 (see link above). The full two-year results, published in the March issue of the Journal of Clinical Investigation, now confirm this original report.

In this two-year, prospective crossover trial, 22 patients were randomised to receive either one year of therapy with recombinant human growth hormone (GH – manufactured by Serono Inc.) followed by one year of follow-up observation (the "GH arm"), or one year of observation followed by one year of GH therapy (the "control arm"). In both arms, the year of GH therapy consisted of daily subcutaneous injections, 3.0 mg daily for six months followed by 1.5 mg daily for the remaining six months. All participants were HIV-positive and on stable antiretroviral therapy, which continued throughout the follow-up period. All but two (who had a viral load of 633 and 272 copies/ml) had undetectable viral loads at baseline. Other median baseline characteristics were: age, 50.3 years, CD4 cell count, 227 cells/mm³; duration of antiretroviral therapy, 2.7 years. The two arms (eleven patients each) were similar in all characteristics; seven patients concluded the two-year GH arm and five concluded the control arm.

After one year of GH treatment, total CD4 cell count increased by 40.1% (95% confidence interval [CI], 8.5–78.9%), versus 10.9% (95% CI, -8.9–49.0%) in observational controls (p = 0.031). Multivariate regression analysis found that the higher CD4 cell percentage was significantly higher in those who received treatment (15.3% greater than untreated, 95% CI, 4.0–27.9%, p = 0.007).

There was also a significantly greater increase in naive CD4 cells with GH treatment: 96.4% (95% CI, 48.6–188.0%) vs. 17.6% (95% CI, -6.2–57.9%) after one year (p = 0.004). This difference remained highly significant in multivariate analysis, with a 62.1% greater gain in naive CD4s with treatment (95% CI, 29.5–102.9%, p < 0.0001). Absolute counts and percentages of CD4 cells and of naive CD4 cells had all continued to increase at three months after discontinuation of GH treatment.

The volume and density of the thymic tissue (markers of healthy thymic cell structure that should be able to produce functional T cells) were also evaluated at baseline, six, and twelve months. At six months, thymic tissue density had increased significantly more in GH-treated participants than in controls (+50 Hounsfield units [HU] vs. +1 HU, p = 0.0005). A benefit, although less dramatic, was sustained at twelve months (+27.5 HU vs. -2.5 HU, p = 0.044); a significant treatment benefit was also retained at both time points in regression analysis (at twelve months: +38.3 HU, 95% CI 15.0–61.6, p = 0.002).

Nearly all (95%) participants experienced grade 2 or greater adverse events with GH treatment, including muscle and joint pain, edema, carpal tunnel syndrome fatigue, insomnia, and blood sugar elevations. The high likelihood of these AEs (typically associated with growth hormone), coupled with its very high cost, make it unlikely that GH will find a place in standard-of-care treatment for HIV infection. However, as the investigators note, these findings "demonstrate that declines in thymic function are reversible in human adults," and that growth hormone is "associated with robust increases in thymic density… and the number of circulating naive CD4+ T cells." This suggests that there might be scenarios in which growth hormone could play a role in enhancing immune recovery.