Th2 cytokines

CYTOKINES, HIV AND AIDS PATHOGENESIS

Deciphering the Th2 conundrum in AIDS: a macrophage perspective L.J. M o n t a n e r and S. Gordon

TH2 cytokines have been presented as the factors responsible for the lack of Thl responses as well as the preferred phenotype of virus-expressing T cells. If there is such substantial evidence against the beneficial effects o f Th2 cytokines, what purpose could its virostatic inhibition in macrophages serve the host? Or is it benefitting the virus? We believe Th2 cytokines may help both virus and host for different reasons. From the point of view of the host, although Th2 cytokines inhibit effective T h l responses, they are also potent regulators which dampen a chronically activated i m m u n e system and reduce the potentially enhancing effects of TNF, ILl and IL6 in virus expression and immune dysfunction. We could postulate that this accounts for the elevated levels of Th2 cytokines seen in HIV-infected patients by Emilie et al. and others irrespective of disease progression. Macrophage-derived I L l 0 has been shown not to be directly related to Th2type responses, serving as an immune deactivating stimulus. Therefore, Th2 cytokines might actually be a factor in reducing viral load if one accepts the correlation between TNFtx-IL I - I L 6 / N F - K b / H I V (see Virelizier and Kaplan, et al.). However, from the point of view of the virus, this effect may be part o f a general mechanisms o f persistence in vivo, since, in addition to reducing T h l responses, the preferential viral production by Th2 T ceils would also inhibit virus in macrophages. This inhibition would subsequently protect newly infected macrophages from any effective i m m u n e clearance of infected Th2 cytokine-producing T cells by CTL. The result would be a renewed expression of virus from newly infected macrophages recovering from the Th2 actions in a TNF, ILl and IL6-rich environment. Therefore, Th2 cytokines should be considered as a potential mediator pathway that is shared between host and virus in responding to an uncontrolled virus-induced activation as well as a mechanism for persistence, respectively. However, we believe the Th2 cytokine pool is not homogeneous and that a preferential usage of the Th2 anti-inflammatory property without directly affecting T h l CD4 differentiation may be possible by the use of ILl3 (see Montaner and Gordon). Thl/Th2 cytokines in AIDS: sorting out the confusion D.J. Blackbourn et al.

The type I/type 2 cytokine balance concept may help to account, at least in part, for the immunopath-

715

ogenesis of AIDS. Type 1 cytokines are suggested by Clerici and Shearer (this Forum) to prevent CD4 ÷ cell loss and dysfunction by programmed cell death (PCD) and type 2 cytokines enhance PCD. However, the type 1/type 2 cytokine profile relationship and its involvement in disease progression in HIVinfected individuals is incompletely understood. Clearly, the TH0 class of cells is involved (Meyaard and Miedema, Romagnani et al., and Vyakamam, this Forum; Maggi et al., 1994). Despite the proviso of the type 1/type 2 relationship being dependent upon the disease stage, of the patient under study and the experimental methods used to analyse cytokine production, the existence of a shift from the type 1 cytokine profile with progression to HIV disease remains controversial (Graziosi et al., 1994 and this Forum). Some of the controversy surrounds the nomenclature that has been used. However, Clerici and Shearer preferentially describe the two cytokine profiles as type 1 and type 2, such that type 1 cytokines promote a c e l l u l a r i m m u n e r e s p o n s e and type 2 a humoral response. This nomenclature then obviates the problem of cytokine production by cells other than those of the TH phenotype which has been discussed elsewhere (Emilie et al., and Graziosi et al., this Forum). Additional confusion exists due to the variety of experimental approaches taken by different research groups to answer the same questions. Thus, opinions differ on the choice of cell type to be studied: mononuclear cells or T-cell clones; antigen-specific or mitogen-stimulated clones derived in the presence or absence of autologous antigen-presenting cells. Furthermore, a diversity of methods has been used to detect either cytokine mRNA or cytokines themselves.

References Maggi et al. (1994), Science, 265, 244-248. Graziosi et al. (1994), Science, 265, 248-252.

General comments on Thl/Th2 cytokines A. Vyakarnam The p a r a d o x i c a l o b s e r v a t i o n s o f T h l / T h 2 cytokines in H1V

It has been known for some time that the cytokine system is dysregulated in HIV (see Poli et al., this Forum). The recent observations of Clerici and Shearer showing a reciprocal relationship between IL2 and IL4 in HIV-infected individuals in different disease stages now suggest that cytokine dysregulation could also affect the balance of Thl/Th2 cytokines in HIV infection.

716

60th F O R U M I N I M M U N O L O G Y

Taking the many observations on T h l f r h 2 cytokines reported in this Forum together, there appear to be two forms of paradoxical observations on the subject: (i) conflicting observations are reported on the pattern of ThlfI'h2 cytokines in HIV and (ii) the cytokines produced by T h l / T h 2 ceils could have opposing effects on virus production depending on the type of cell infected. Conflicting observations on the pattern o f Thl/Th2 cytokines in HIV

In general, there is c o n s e n s u s a m o n g m a n y groups (though not all) that HIV infection results in the upregulation of many cytokines. In particular, TNF, IFN gamma and IL6 tend to be overproduced and indeed, in this Forum there are data from several groups showing increased IFN), in tissue as well as in PBMC (Emilie et al., Graziosi et al.). On the other hand, past data suggests that IL2 production is impaired in HIV patients even before the decline of CD4 + T cells ( M i e d e m a et al. (1988), J. Clin. Invest., 82). The question of IL4 production in HIV is, however, hotly debated. Three groups in this Forum report increased production of IL4 in a proportion of HIV patients: Clerici and Shearer (PBMC level), Romagnani (T-cell clones) and Meyaard et al. (T-cell clones). However, two other groups have failed to detect increased ILA tissues (Emilie et al. and Graziosi et al.). We suggest that this discrepancy may be due to the following reasons: firstly, the number of HIVpositive individuals analysed by the two groups, who either failed to detect IL4 in tissues (Emilie et al.) or found no differences between patients and controls (Graziosi et al.), is considerably smaller than that analysed by Clerici and Shearer, who analysed cytokine patterns in over 100 patients at the PBMC level. As with most cytokine analysis, intragroup variation in cytokine levels could mask any differences in cytokine production between groups, especially if the differences in cytokine levels are small (which could still be biologically relevant) or i f the g r o u p s a n a l y s e d are s m a l l in n u m b e r s . Secondly, our data and that of Romagnani showing that HIV replicates more efficiently in IL4-producing Th2/Th0 clones in contrast to T h l clones suggests that in sites of immune activation and active virus production such as lymph nodes, ILA-producing cells may be preferentially lost. Finally, the capacity of HIV to replicate efficiently in IL4-producing cells may be linked to the capacity of the virus to activate these cells to produce cytokines which support its replication in an autocrine manner. Analogous observations have been reported in the past linking HIV replication in T ceils to the autocrine production of cytokines like TNF (see Poli et al., this Forum). The active replication of HIV in

IL4-producing cells could therefore explain not only why these cells may be lost in sites of active virus production, but also why these ceils could become activated leading to the overproduction of IL4. Taking these observations together, the problem that one is faced with in HIV is the marked reduction in the production of the T h l cytokine IL2 and the gross overproduction of IFN)'. Whilst impaired IL2 production may represent defects at the level of CD4 + T cells (either defects in the biochemical pathways in T-cell activation or inhibition due to the presence o f cross-regulatory T h 2 - t y p e cytokines like IL4) or at the level of APC which fail to efficiently trigger CD4 ÷ T ceils, the overproduction o f IFNT points strongly not so much to the o v e r a c t i v a t i o n o f T h l cells which produce this cytokine as to overactivated CD8 ÷ T cells (most of the cells positive for this cytokine, especially in the tissues, are CD8+; see Emilie et al.). Indeed, we would argue that the loss of IL4-producing cells due to their capacity to efficiently support HIV replication may partly account for the overproduction of IFNT. The cytokines produced by Thl/Th2 cells could have opposing effects on virus production depending on the type o f cell infected

Why is it that IL4-producing T cells supports HIV replication more efficiently than T h l - t y p e T cells (Vyakarnam, Romagnani, this Forum) when separate studies show IL4 to be inhibitory for I/IV replication in macrophages (Montaner and Gordon, Schuitemaker). The answer to this question is not known, although it has been shown that some cytokines like TNF, for example, could have opposing effects in a given disease because of their ability to both directly affect the replication of a pathogen or by secondary effects on other cells of the immune system or by opposing effects on opportunistic infections which could accompany a given disease (see Vassalli, P. (1992), Annu. Rev. Immunol., 10, 411). In view of the considerable redundancy of the cytokine system and the multiple cytokines produced by a group of cells in a microenvironment, the potency of a cytokine on HIV would depend on a hierarchy of cytokine effects on the virus. Devising experimental systems which attempt to address the hierarchy of multiple cytokines in cell mixtures is not easy, but could be facilitated by studies to assess the role of a cytokine in cell populations chosen for their homogeneity in terms of cytokine production. This may be feasible at least at the level of the CD4 and the CD8 + T-cell populations which can be cloned in vitro. We can confirm, for example that differential virus replication in CD4 + T cells which do and do not produce ILA was not detected until such

CYTOKINES, H I V A N D A I D S P A T H O G E N E S I S cells were cloned. Using these defined populations, it would now be possible to define the hierarchy of the cytokines produced by each population in terms of their effects on HIV.

HIV pathogenesis: Th2 shift, Th2 CD8 + T cells and CD30 S. Romagnani, E. Maggi and G. Del Prete Our results do not exclude the possibility of a Th2 shift in vivo during HIV infection, but rather provide a reasonable explanation for the difficulty of demonstrating its occurrence in vitro. Although failing to demonstrate increased IL4 production in vitro by peripheral blood mononuclear cells of HIV-infected individuals, our paper makes three major points. First, the important T-cell population specific for recall antigens did show a moderate shift toward the expression of Th cells producing both Thl- and Th2-type cytokines (i.e. ThO cells). The second point is that there was a selective depletion of CD4 ÷ Th ceils inducible to the production of Th2-type cytokines in the advanced phases of HIV infection. Finally, HIV was found to replicate preferentially in Th2 and Th0 rather than T h l clones in vitro. The latter finding may reconcile the apparently conflicting results of different studies. Indeed, if there is a balance during infection between the induction of Th2 bias and rapid killing of Th2 and Th0 cells at the lymph node level, where the HIV burden is demonstrable, high levels of Th2 cytokines may not be seen because of the early death of infected T h 2 f r h 0 cells. This hypothesis was recently substantiated by a new series of data (part of which are not yet published). First, we were able to generate high numbers of CD8 ÷ T-cell clones (which are resistant to HIV infection and therefore to direct HIV-mediated killing) showing an unusual clear-cut Th2-1ike profile of cytokine secretion from both peripheral blood and skin of HIV-infected patients with low levels of circulating CD4 ÷ T cells. Secondly, we found that CD30, a m e m b e r o f the T N F r e c e p t o r s u p e r f a m i l y , is expressed on both CD4 ÷ and CD8 ÷ T-cell clones producing Th2-type cytokines, but not in Thl-like cells. More importantly, HIV was able to promote CD30 expression in vitro and CD30 triggering in HIV-infected ceils significantly enhanced HIV replication and CD4 ÷ T-cell death. Accordingly, Pizzolo et al. have recently shown increased serum levels of soluble CD30 (sCD30), which reflects the bulk of cells expressing CD30 in vivo at first evidence of HIV infection in the great majority of HIV-seropositive individuals. The increased sCD30 levels did represent a prognostic factor independent of other prognostic parameters, including the initial absolute number of circulating CD4 ÷ T cells. Taken

717

together, these data suggest that HIV induces high CD30 turnover in T cells (which is associated with the production of Th2 cytokines), while CD30 triggering favours HIV replication and CD4 ÷ T-cell death. The clarification of the molecular mechanisms involved in this complex and fascinating link between the production of Th2 cytokines, CD30 expression/sCD30 release and HIV replication, will probably contribute to a better understanding of the immunopathogenesis of HIV infection.

Overview of cytokine climate, CD8 T cells and therapies in AIDS P.L. Salk and J. Salk I. Are c h a n g e s in the Type .l/Type 2 c y t o k i n e "climate" involved in AIDS pathogenesis ? Clerici and Shearer have sum m ari zed their large body of work which suggests that a THl-like (Type 1 ; see Bloom et al., 1992) predominant immune response might protect against the establishment of progressive HIV infection, and that in individuals with progressive infection, Type 1 responses decline and TH2-1ike (T ype 2) responses increase. Two characteristics of their studies may be important for the purpose of comparison with o t h e r s t u d i e s . First, H I V - i n f e c t e d individuals have been staged with respect to functional lymphocyte defects rather than CD4 ÷ T-cell levels. In this regard, it should be noted that the increase in IL4 production observed in their studies occurred only in the subset of subjects whose l y m p h o c y t e s had lost their r e s p o n s i v e n e s s to recall antigens, but not to alloantigens or mitogens, and that I L l 0 production increased as a function of the severity of the lymphocyte functional defects, but did not correlate with the clinical stage of the patients (Clerici et al., 1994). Second, lymphokine production was evaluated in vitro in mixed cell populations (peripheral blood mononuclear cells; PBMC) stimulated with either antigens or mitogen (PHA). Another aspect of their studies that is important is the finding that lymphocyte functional defects could be reversed in vitro by the Type 1 cytokine ILl2 or by anti-IL4 or anti-ILl0 antibodies. Furthermore, they have found that Type 2 cytokines promote programmed cell death (PCD) in T cells from HIV-infected individuals, whereas T y p e 1 cytokines inhibit PCD. Other studies reported in this volume are in accord with the concept that changes in the balance of Type 1 and Type 2 cytokines occur in association with HIV infection. Meyaard and Miedema reported that T-cell clones from two out of three HIV-infected individuals showed a decrease in the