CLINICAL USE OF CYTOKINES IN HIV INFECTION
IFN-a∕β has shown potent antiviral activity in in vitro and ex-vivo infected cells, animal models of HIV infection as well as in vivo. In HIV-infected SCID mice reconstituted with human cells or tissues IFN-α, provided by i) continuos infusion or either expressed by ii) U937 cells or iii) CD4+ T cells containing a retroviral vector expressing the cytokine, strongly reduced infection and protected from HIV-induced CD4+ T cell depletion [329].
The anti-viral mechanism of IFN-α observed in the animal model has been associated with trapping of new progeny virions within cells, resulting in anti-HIV potency and protection from virus- induced depletion of CD4+ T cells higher than what observed with AZT [329].In vivo, administration of IFN-α as a therapeutic agent has been proven to have beneficial effects in either AIDS patient either Kaposfsarcoma and high (>500 cells∕μl) CD4+ T cell counts [330,331], or in individuals with HIV-associated progressive multifocal leukoencephalopathy. Moreover, the polyethilen-glycol (PEG) conjugated form of IFN-α, characterized by a prolonged half-life, has been shown to enhance lytic activity along with increased concentrations of perforin and granzyme A expression in NK cells were demonstrated in HIV+ individuals [332].
IL-2. As observed in SCID-hu mice [333], expression of IL-2 by a vaccinia virus vector or by means of DNA vaccination did not alter viral replication, but proved to represent a good adjuvant [334]. Due to the short halflife of IL-2, a chimeric IL-2∕Ig fusion protein has been developed and has shown to increase the efficacy of DNA vaccination also in terms of CD4+ T cells activation and proliferation [335-337]. In addition, IL-2 has been shown to prevent depletion of immature CD4+CD8+ and CD5+CD1+ thymocytes in the HIV-infected thymus of SCID mice implanted with human fetal thymus and liver tissues without increasing viral load [333].
In addition, IL-2 administration in humans has been shown to induce HIV peptide-specific and nonspecific IFN-γ producing cells [338], and both naive [257], memory T cells [339] and NK cells [340].IL-2 did not increase viral production, but strongly synergized with IL-4 in cultures of infected mature thymocytes [64]. IL-2 and IL-4 induced over-expression of CCR5 and CXCR4 [91], and increased viral replication also in HIV-infected tonsils [65]. In thy/liv-SCID-hu mice, exogenous administration of IL-2 was demonstrated to maintain alive an immature thymocyte subset without increasing viral load [341]. Furthermore, IL-2 prevented the outgrowth of human Epstein-Barr virus (EBV)-induced lymphoproliferative disease (an opportunistic cancer typical of AIDS patients) in hu-PBL-SCID mice engrafted with human PBL from healthy EBVseropositive donors [342]. In this regard, IL-2 has been successfully used in humans to prevent EBV disease [343,344].
IL-2 plus HAART reduced the state of immune activation and, in some studies, the HIV DNA content of PBMC, likely as a consequence of the numerical expansion of circulating CD4+ T cells [345]. In addition, administration of IL-2 without antiretrovirals has not resulted in the expression of a previously silent quasispecies [346]. Administration of intermittent IL-2 induced “blips” of virus replication without inducing an increase in steady-state viremia, even in the absence of a full suppression of HIV replication by HAART [347].
Unique among all cytokines, IL-2 has been evaluated in phase III clinical trials (ESPRIT and SILCAAT) for its potential therapeutic effect when administered with anti-retroviral therapy to stably increase the levels of circulating CD4+ T cells [348]. In spite of promising conclusions of phase II studies and of their metaanalysis, these trials have recently unequivocally demonstrated the lack of efficacy of intermittent IL-2 administration in terms of protection from AIDS-related deaths and events defining AIDS progression in the face of significantly increased levels of peripheral CD4+ T cell counts [349,350].
Furthermore, increased cardiovascular pathologies, were linked to IL-2 administration [351]. The reasons for this unexpected clearcut failure of this promising form of immunotherapy are unclear and will be throroughly investigated in the next years.It is likely that the net effect of cytokines and chemokines in in vivo infection will result not only from a direct effect on the infected cells, but also, and perhaps even more importantly, from the indirect activation of other effectors, such as CD8+ T and NK cells, which may contribute and even take advantage of transiently increased expression of viral proteins to eliminate infected cells. These concepts are currently explored in experimental protocols aimed at eradicating infection from latent reservoirs, including resting memory T cells [352,353] and monocytes [354,355].
Finally, IL-2 has also been used to ameliorate clinical side-effects induced by HAART (see paragraph about IRIS).
IL-12 administration did not induce substantial changes in plasma viremia and viral load in PBMC of rhesus macaques chronically infected with SIV [45]. However, an increased proliferative response of T cells to multiple HIV Ag was noted in chimpanzees vaccinated with HIV DNA plus IL-12 [356]. Administration of exogenous IL-12 did not affect either the virus load or the frequency of circulating infected lymphocytes in chronically SIV-infected rhesus monkeys [45], although it increased the proliferative response to multiple HIV Ags in chimpanzee vaccinated with a DNA-based vaccine [357]. IL-12 administration to either SIV- infected macaques [45] or HIV-infected individuals [356] at an early stage of infection increased the frequency and activity of circulating NK cells in contrast to an earlier report indicated a paradoxical decrease of NK cell activities in IL-12 treated HIV+ individuals [358].
IL-15 has been shown to exert number of immunorestorating functions, including the up-regulation of Ab- dependent cellular cytotoxicity from PBMC and several polymorphonuclear phagocyte functions in HIV+ individuals [116,130].
In addition, IL-15 induced virus-specific CTL in primates infected with SIV and in humans [359]. Exogenous IL-15 restored the deficient IL-12 production from ex-vivo infected PBMC [117]; in contrast, neither IL-15 nor IL-12 reversed the functional anergy of γδ T cells isolated from infected individuals [360]. IL-15 stimulation of NK cells, from both HIV+ or HIV seronegative individuals, induced their proliferation and cytokine production [40,117,120,361-363]. These effects, together with the IL-15 ability to boost NK cell functions and PBMC proliferation in response to a number of Ag in synergy with IL-2 [364], renders this cytokine of great interest for clinical exploitation.IL-15 has been also combined to IL-21 resulting in a synergistic expansion and functional enhancement of CD8+ T cells both in vitro and in vivo in mice [172,173], as well as on CD8+ T cells harvested from HIV+ individuals [174]. For the above reasons, IL-15 has been tested as a vaccine adjuvant for experimental SIV infection with contrasting results. Early reports indicated that IL-15 enhanced cell-mediated and humoral immune responses in chronically SIV-infected macaques [365] as well as in macaques immunized with SIVgag DNA resulting in an ameliorated clinical outcome after virus challenge [366]. In contrast, recent studies indicated that IL-15 did not ameliorate the immune responses of macaques immunized with ALVAC-SIV-gpe expressing the gag, pol, and env genes of SIVmac25ι [367] and even abrogated the vaccine-induced decrease of the viral set point [367] while not influencing the levels of viremia in chronically infected macaques [365].
TNF-a. Because TNF-α∕β are able to inhibit R5 HIV entry and because of their ability to kill preferentially cells infected by several viruses, particularly in combination with IFN-γ [368,369], TNF-α∕β were originally considered potential anti-viral agents. TNF-α may indeed play a protective role on uninfected cells by enhancing the capacity of NK/LAK cells to kill cells chronically infected with HIV [370], although stimulation of MDM after infection confirmed that up-regulation of viral expression occurred [152]. In contrast, TNF-α induced the expression of CXCR4 and replication of X4 viruses in both U937 cell clones [371] and PBMC stimulated with anti-CD3 plus anti-CD28 Ab [372]. These observations suggest that the increased levels of TNF-α∕β observed in the advanced stages of infection may play a role in the emergence of CXCR4-using viruses.