IL-18 underexpression reduces IL-2 levels during HIV infection: a critical step towards the faulty cell-mediated immunity?

Field: Medicine (Health Sciences)

IL-18 is one of the most recently characterized cytokines. Like IL-12, IL-18 is mainly produced by monocytes/macrophages, and is a potent inducer of the cell-mediated immune responses [1,2]. Moreover, IL-18 is able to activate T lymphocytes to synthesize IL-2 [1]. Monocytes/macrophages are the first HIV-infected cells and the pattern of monokine secretion is profoundly affected (including an underexpression of IL-12) [3]. The reduced production of IL-2 by CD4 T lymphocytes is also a major characteristic of HIV infection [4], appearing very early. More generally, cell-mediated immunity is seriously deficient in the disease. These facts prompted us to measure IL-18 levels in a group of HIV-infected patients. We selected a group of eight HIV-infected patients at intermediate stages with detectable viral loads and decreased CD4 cell counts. The median plasma viral load was 4.09 log copies/ml (quartiles: 2.62–5.10) and median CD4 cell counts were 264 per mm3 (quartiles: 159–369) on the day of blood sampling. All the patients received reverse-transcriptase inhibitors (given at standard doses). Other characteristics were: median age, 35 years (quartiles: 32–37); sex, one woman and seven men; median HIV infection length, 7 years (quartiles: 6–10). As the IL-18 gene is constitutively transcribed [2], we measured IL-18 messenger RNA levels in unstimulated, freshly isolated peripheral blood mononuclear cells from these patients and from healthy donors, as a control (see Fig. 1).Fig. 1.: Venous blood was collected with sodium heparin as an anticoagulant. On the day of blood sampling, peripheral blood mononuclear cells were isolated by Ficoll–Hypaque sedimentation, and total RNA was obtained by the RNAzol method. mRNA of IL-18 and hypoxanthine phosphoribosyltransferase (HPRT) were measured by quantitative reverse transcriptase–polymerase chain reaction using exponential phase and external standards (plasmid references) followed by a non-radioactive hybridization, as described in detail elsewhere [5]. Oligonucleotides used for polymerase chain reactions and hybridizations were obtained by the Oligo Primer Analysis software (National BioSciences, Inc., USA). The result is expressed as the IL-18 : HPRT ratio for each individual. (a) The individual IL-18 : HPRT mRNA ratios of the HIV-infected patients (HIV+, n = 8) are plotted with those of a group of healthy adult donors (HIV−, n = 11). The boxes enclose quartiles and the central bars represent median values. The difference between the two groups is statistically significant (P < 0.04, non-parametric Mann–Whitney test). (b) For each patient, the IL-18 : HPRT mRNA ratio is plotted with the IL-2 : HPRT mRNA ratio. The correlation between these two values is significant (r' = 0.86, P < 0.03, non-parametric Spearman test).Fig. 1a shows that IL-18 was less highly expressed in patients than in controls. The median value of the IL-18 : hypoxanthine phosphoribosyltransferase (HPRT) mRNA ratio was 0.69 in the control group whereas it was only 0.22 in the patient group (P < 0.04, non-parametric Mann–Whitney test). This corresponded to a 70% decrease. As IL-18 is able to induce IL-2, we also measured IL-2 mRNA levels in the same patients. Fig. 1b represents for each patient both IL-2 and IL-18 expressions. When statistically analysed, these two parameters showed a significant positive correlation (r' = 0.86, P < 0.03, non-parametric Spearman test). This indicates that both mRNA levels were linked in this group of patients. In conclusion, these results suggest that there is a defect in IL-18 expression during HIV infection, and that this underexpression represents at least one mechanism explaining the defect in IL-2 production during HIV infection. The underexpression of IL-18 shown here becomes very significant when added to the previously known IL-12 hyposecretion in this disease, because both these monokines play a triggering role in the initiation of cell-mediated immunity [1,3]. Furthermore, IL-18 deficiency may be more pivotal than IL-12 deficiency because IL-18 also induces IL-2 secretion, whereas IL-12 does not [1]. The decreased IL-2 production in HIV infection certainly plays a critical role in the failure of the immune system, because IL-2 amplifies the extent of many immune responses. Finally, these results, which furnish an explanation for deficient cell-mediated immunity, should also be considered when analysing the future of HIV immunotherapy. Acknowledgements This work would not have been possible without the daily assistance of S. Gidrol and H. Keller. The authors would like to thank Professor T. Taniguchi (University of Tokyo, Japan), Professor P. Kourilsky (Institut Pasteur, Paris), and Dr A.M. Jackson (University of Leeds, UK) for the kind gifts of some very useful reagents. They are also grateful to Dr X. Di, M. Azar, J.-L. Lagneau, and T. Péréa for their invaluable work. Denis Davida Danièle Chevrierb Marie-Pierre Treilhouc Marcel Joussemetd Bertrand Dupontc Jacques Thèzea Jean-Luc Guesdonb