CLINICAL SIGNIFICANCE OF CD8+ T CELLS AND SOLUBLE ANTIVIRAL FACTORS.

Influence of Highly Active Antiretroviral Therapy (HAART) on CAF-Mediated Antiviral Activity and its Implications in Anti-HIV Treatment.

It has been shown that [68] the suppressor activity of CAF during antiretroviral therapy was associated with the loss of viremia.

Previously, Gray CM et al. [70] have also shown a strong correlation between reduced viral load and loss of activated CD8+CD38+HLA-DR+ cell numbers in the absence of protease inhibitors. Only dual therapy was used in the study of ten HIV-infected asymptomatic patients. These studies are further supported by data from Stanford SA et al. [71], which showed the impact of HAART on the plasma HIV-1 RNA levels, CD4+ and CD8+ T lymphocyte counts, and the CD8+ T cell anti-HIV response.

Recently, Torres KJ [73] investigated whether HAART has different effects on CNAR in patients at the intermediate and late stages of HIV infection. They studied untreated healthy HIV-infected subjects at baseline and regular intervals for at least 48 weeks following initiation of HAART. Baseline CNAR activity in all subjects correlated inversely with viral load and directly with CD4 T+ cell counts. The level of CNAR in the late stage group was significantly lower than in the intermediate-stage and the healthy reference group (p < 0.01). Following initiation of HAART, CNAR activity was increased significantly during HAART, but only in the late-stage group (p < 0.01). This increase in CD8+ T cell function was seen within 4 weeks of treatment initiation and resulted in levels of CNAR activity almost equal to those observed in the healthy reference subjects. Their findings suggest a beneficial effect on CNAR in those individuals with reduced activity, especially in late-stage infection. But they do not clarify, if HAART may also have some adverse effect on CD8-mediated antiviral inhibition in patients who are doing well.

How Protease Inhibitors Affect Soluble Factor-Mediated Antiviral Activity of CD8+ T Cells?

As discussed above that HAART therapy may have some bearing on CD8+ T cell antiviral activity, recent studies by Stanford et al. [71] and Wilkinson et al. [68], have shown that HAART has an effect on CD8+ T cell non-cytotoxic antiviral activity, but it is unclear whether some anti-HIV drugs have a deleterious effect on this antiviral activity. One of the protease inhibitors leupeptin, which is not a prescribed drug in the HAART regimen for treatment, was shown to block CNAR activity by up to 95%, in vitro. The effect was shown to be dose-dependent and was observed in up to 70% of the CAF and CNAR assays by using fluids and cells from several different subjects. Pretreatment of CD8+ T cells with leupeptin reduced CNAR, further supporting an inhibitory effect on a CD8+ T cell product. Leupeptin did not affect cell growth, expression of activation antigens, or viability of cells (both CD4+ and CD8+ T lymphocytes). Thus it appears that a part of the CD8+ T cell non-cytotoxic response may involve the activity of a protease or a protein that is capable of interacting with certain protease inhibitors, in particular leupeptin [58]. In light of these data, an assessment of the effect(s) of each of the protease inhibitors and combination drugs on CNAR activity needs to be evaluated, as it may provide strategies to maintain this activity and lead to better immune restoration during HAART therapy. In turn, this may also provide better long-term clinical management of HIV patients and may possibly provide better drug choices.

Relationship Between CD4+ T Cell Depletion and Failure of CD8+ T Cell to Combat HIV

During primary infection, HIV-1 infects CD4+T cells, as well as cells of monocyte/macrophage lineage through the mucosal tracts. In this period, neutralized infecting virions as well as the selection of resistant variants and latently infected pool of CD4+ T cells are established [32].

The thymus is one of the sites for T cell production in the adult life. When HIV-1 is established within the thymic epithelium and the remainder of the thymic stroma, the specific cell populations affected may interfere with the selection process during the development of T cells, leading to immune suppression/tolerance caused by clonal deletion, clonal anergy and/or clonal suppression [77]. Thus, the impaired production in the thymus leads to the progressive shrinking of memory CD4+ T cell pool.

CD4+ T cell help is essential for both the full function and the maintenance of CD8+ T cell responses. This help is essential for the naive CD8+ precursor CTL to differentiate into mature CTL [78]. This help may depend on cytokine secretion to some extent [79]. In addition, it has been shown that activated CD4+ T cells specifically interact with DC through CD40L-CD40 binding, hence activating DC into a potent inducer of the CTL response [80, 81]. It has been observed that the absence of CD4+ T cells is associated with a lower level of IFN-γ production by HIV-specific CTL [82], impaired CTL function and the wane of CTL activity in the late stages. Vigorous CTL responses are associated with high levels of CD4+ T cells [83].

response when transferred into a CD4+ T cell deficient host.

It has been noted that HIV-1-specific CD8+ T cells lose the ability of proliferation with ongoing viral replication after acute infection. Lichterfeld [87] has shown that this functional defect can be induced in vitro by depletion of CD4+ T cells. Further, it can be corrected during chronic infection in vitro by addition of autologous CD4+ T cells isolated during acute infection and in vivo by vaccine-mediated induction of HIV-1- specific CD4+ T helper cell responses. These data demonstrate that HIV-1-specific proliferation of CD8+ T cells critically depends on the presence of antigen-specific CD4+ T cells.

Clinical Correlation of CD8+ T Cells with Plasma Viremia and Proviral DNA in HIV Patients

Acute HIV-1 infection leads to rapid expansion of HIV-1-specific CD8+ T cells [88], which is followed by a rapid and dramatic decline of the viremia [42, 89]. In chronic infection, stronger and broadly diversified HIV-1- specific CD8+ T cell responses have been detected, yet the viremia remains high [46, 90, 91]. Hazenberg et al. [92] have carried out a longitudinal analysis in patients before and during HARRT to investigate the proliferation in peripheral blood CD4+ and CD8+ T cells by detecting the Ki-67 nuclear antigen expression [92]. In untreated HIV-1 infection, the percentage and number of Ki-67+CD8+ T cells were significantly increased compared with values from healthy individuals.

Further, an elegant study by Salerno-Goncalves et al. [97] has measured CD8 antiviral activity in 22 asymptomatic human immunodeficiency virus type 1-infected patients (10 rapid progressors and 12 slow progressors) and its effect on the proviral load of CD4+ T cells homogeneously superinfected by the same dose of a non-syncytium-inducing virus in the presence or in the absence of autologous CD8+ T cells. They demonstrated that the antiviral activity of CD8+ T cells was highly predictive of the rate of peripheral CD4+ T-cell decline and reducing proviral DNA integration in autologous CD4+ T cells. These findings show a strong correlation between the antiviral activity of CD8+ T cells of HIV-infected patients (as measured by the CD4+ T-cell proviral DNA decrease) and the rate of peripheral CD4+ T-cell count decline in the next 3 years. This proviral DNA in rapid and slow progressors correlated clinically with plasma viremia levels. They also imply that such an activity could be the sum of various HIV-specific CD8+ T-cell activities, but they failed to find any correlation with the Gag-specific CTL activity.

Secretion of CD8 Soluble Antiviral Factor and HIV Disease Staging and Balance Between CD4+ and CD8+ T Cell Counts

CD4/CD8 T cell ratio is a more sensitive predictor of HIV infection than the CD4 T cell count alone Thus CD4/CD8 T cell ratios may prove to be vital in predicting the CD8 antiviral activity in HIV patients and SIV- infected macaques. Recently, it has been shown through a cross-sectional study that the capacity of CD8+ cells from HIV-infected patients and SIVmac-infected macaques to suppress the replication in vitro depends on the clinical stage of disease [98]. Little is known about changes in this antiviral activity over time in individual HIV-infected patients or SIV-infected macaques. Dioszeghy assessed changes in the soluble factor- mediated non-cytolytic antiviral activity of CD8+ cells over time in eight cynomologus macaques infected with SIVmac251 to determine the pathophysiological role of this activity. They found that CD8+ cell- associated antiviral activity increased rapidly in the first week after viral inoculation and remained detectable during the early phase of infection. This net increase in antiviral activity of CD8+ cells correlated with plasma viral load throughout the 15 months of follow-up. CD8+ cells gradually lost their antiviral activity over time and acquired virus replication-enhancing capacity. Levels of antiviral activity also correlated with the total CD4+ T-cell counts after viral set point. Further, the beta-chemokines and interleukin-16 and alpha defensins in CD8+ cell supernatants did not correlate with CD8 antiviral activity. More importantly, the soluble factor- mediated antiviral activity of CD8+ cells was neither cytolytic nor restricted to major histocompatibility complex. This is the first longitudinal study to demonstrate that the increase in non-cytolytic antiviral activity from baseline and the maintenance of this increase over time in cynomolgus macaques depend on both viral replication and CD4+ T cell count, implying the significance of balance in ratio of CD4+ and CD8+ T cells in SIV-infected monkeys and HIV-infected humans.