Interleukin-13: characterization and biologic properties

A key role for IL-13 signaling via the type 2 IL-4 receptor in experimental atopic dermatitis

IL-13 and IL-4 are potent mediators of type 2-associated inflammation such as those found in atopic dermatitis (AD). IL-4 shares overlapping biological functions with IL-13, a finding that is mainly explained by their ability to signal via the type 2 IL-4 receptor (R), which is composed of IL-4Rα in association with IL-13Rα1. Nonetheless, the role of the type 2 IL-4R in AD remains to be clearly defined. Induction of two distinct models of experimental AD in Il13ra1 ^-/- mice, which lack the type 2 IL-4R, revealed that dermatitis, including ear and epidermal thickening, was dependent on type 2 IL-4R signaling. Expression of TNF-α was dependent on the type 2 IL-4R, whereas induction of IL-4, IgE, CCL24, and skin eosinophilia was dependent on the type 1 IL-4R. Neutralization of IL-4, IL-13, and TNF-α as well as studies in bone marrow-chimeric mice revealed that dermatitis, TNF-α, CXCL1, and CCL11 expression were exclusively mediated by IL-13 signaling via the type 2 IL-4R expressed by nonhematopoietic cells. Conversely, induction of IL-4, CCL24, and eosinophilia was dependent on IL-4 signaling via the type 1 IL-4R expressed by hematopoietic cells. Last, we pharmacologically targeted IL-13Rα1 and established a proof of concept for therapeutic targeting of this pathway in AD. Our data provide mechanistic insight into the differential roles of IL-4, IL-13, and their receptor components in allergic skin and highlight type 2 IL-4R as a potential therapeutic target in AD and other allergic diseases such as asthma and eosinophilic esophagitis.

Development of new antituberculous drugs based on bacterial virulence factors interfering with host cytokine networks

The worldwide increase in the prevalence of tuberculosis (TB), especially multidrug-resistant TB and extensively drug-resistant TB, is an important global health concern, and new effective drugs are urgently needed. Information on the genome of Mycobacterium tuberculosis (MTB) and various mycobacterial virulence genes is leading to the identification of genes that code for new drug targets. Mycobacterium tuberculosis (MTB) is resistant to the antimicrobial mechanisms of host macrophages and can survive and replicate in macrophages for long periods, resulting in a persistent infection. Mycobacterial virulence factors suppress macrophage bactericidal functions partly via their downregulatory effects on the host antimicrobial cytokine networks, consisting of proinflammatory, immunopotentiating, and Th1-inducing cytokines. Thus, for the development of unique drugs that exhibit antimycobacterial action through novel mechanisms, it is reasonable to search for targets among bacterial genes encoding virulence factors which interfere with the host cytokine responses protective to mycobacterial pathogens. In this review, we discuss the profiles of cytokine networks related to host resistance to mycobacteria, including the mechanisms of downregulation of host antimycobacterial immunity due to immunosuppressive cytokines, which are occasionally induced in the advanced stages of TB. We also highlight the development of antituberculous drugs based on bacterial virulence factors interfering with the host antimycobacterial cytokine network.

Intracranial elimination of human glioblastoma brain tumors in nude rats using the bispecific ligand-directed toxin, DTEGF13 and convection enhanced delivery

A bispecific ligand-directed toxin (BLT) consisting of human interleukin-13, epithelial growth factor, and the first 389 amino acids of diphtheria toxin was assembled in order to target human glioblastoma. In vitro, DTEGF13 selectively killed the human glioblastoma cell line U87-luc as well as other human glioblastomas. DTEGF13 fulfilled the requirement of a successful BLT by having greater activity than either of its monospecific counterparts or their mixture proving it necessary to have both ligands on the same single chain molecule. Aggressive brain tumors established intracranially (IC) in nude rats with U87 glioma genetically marked with a firefly luciferase reporter gene were treated with two injections of DTEGF13 using convection enhanced delivery resulting in tumor eradication in 50% of the rats which survived with tumor free status at least 110 days post tumor inoculation. An irrelevant BLT control did not protect establishing specificity. The bispecific DTEGF13 MTD dose was measured at 2 microg/injection or 0.5 microg/kg and toxicity studies indicated safety in this dose. Combination of monospecific DTEGF and DTIL13 did not inhibit tumor growth. ELISA assay indicated that anti-DT antibodies were not generated in normal immunocompetent rats given identical intracranial DTEGF13 therapy. Thus, DTEGF13 is safe and efficacious as an alternative drug for glioblastoma therapy and warrants further study.

IL-4/IL-13-dependent alternative activation of macrophages but not microglial cells is associated with uncontrolled cerebral cryptococcosis

Both interleukin (IL)-4- and IL-13-dependent Th2-mediated immune mechanisms exacerbate murine Cryptococcus neoformans-induced bronchopulmonary disease. To study the roles of IL-4 and IL-13 in cerebral cryptococcosis, IL-4 receptor alpha-deficient (IL-4Ralpha(-/-)), IL-4-deficient (IL-4(-/-)), IL-13-deficient (IL-13(-/-)), IL-13 transgenic (IL-13(T/+)), and wild-type mice were infected intranasally. IL-13(T/+) mice displayed a higher fungal brain burden than wild-type mice, whereas the brain burdens of IL-4Ralpha(-/-), IL-4(-/-), and IL-13(-/-) mice were significantly lower as compared with wild-type mice. On infection, 68% of wild-type mice and 88% of IL-13-overexpressing IL-13(T/+) mice developed significant cerebral lesions. In contrast, only a few IL-4Ralpha(-/-), IL-4(-/-), and IL-13(-/-) mice had small lesions in their brains. Furthermore, IL-13(T/+) mice harbored large pseudocystic lesions in the central nervous system parenchyma, bordered by voluminous foamy alternatively activated macrophages (aaMphs) that contained intracellular cryptococci, without significant microglial activation. In wild-type mice, aaMphs tightly bordered pseudocystic lesions as well, and these mice, in addition, showed microglial cell activation. Interestingly, in resistant IL-4(-/-), IL-13(-/-), and IL-4Ralpha(-/-) mice, no aaMphs were discernible. Microglial cells of all mouse genotypes neither internalized cryptococci nor expressed markers of alternative activation, although they displayed similar IL-4Ralpha expression levels as macrophages. These data provide the first evidence of the development of aaMphs in a central nervous system infectious disease model, pointing to distinct roles of macrophages versus microglial cells in the central nervous system immune response against C. neoformans.

Distinct roles for IL-13 and IL-4 via IL-13 receptor alpha1 and the type II IL-4 receptor in asthma pathogenesis

IL-13 and IL-4 are central T helper 2 (Th2) cytokines in the immune system and potent activators of inflammatory responses and fibrosis during Th2 inflammation. Recent studies using Il13ra1(-/-) mice have demonstrated a critical role for IL-13 receptor (IL-13R) alpha1 in allergen-induced airway responses. However, these observations require further attention especially because IL-4 can induce similar lung pathology to IL-13, independent of IL-13, and is still present in the allergic lung. Thus, we hypothesized that IL-13Ralpha1 regulates IL-4-induced responses in the lung. To dissect the role of IL-13Ralpha1 and the type I and II IL-4Rs in experimental asthma, we examined lung pathology induced by allergen, IL-4, and IL-13 challenge in Il13ra1(-/-) mice. We report that IL-13Ralpha1 is essential for baseline IgE production, but Th2 and IgE responses to T cell-dependent antigens are IL-13Ralpha1-independent. Furthermore, we demonstrate that increased airway resistance, mucus, TGF-beta, and eotaxin(s) production, but not cellular infiltration, are critically dependent on IL-13Ralpha1. Surprisingly, our results identify a CCR3- and IL-13Ralpha1-independent pathway for lung eosinophilia. Global expression profiling of lungs from mice stimulated with allergen or IL-4 demonstrated that marker genes of alternatively activated macrophages are differentially regulated by the type I and type II IL-4R. Taken together, our data provide a comprehensive mechanistic analysis of the critical role by which IL-13Ralpha1 mediates allergic lung pathology and highlight unforeseen roles for the type II IL-4R.

Anti-glioblastoma effect of a recombinant bispecific cytotoxin cotargeting human IL-13 and EGF receptors in a mouse xenograft model

To improve activity of a recombinant IL-13 cytotoxin (CT) comprised of IL-13 spliced to truncated diphtheria toxin (DT(390)), epidermal growth factor (EGF) was added to the same single chain protein. This new recombinant bispecific CT, called DTEGF13, enhanced the killing potency against the human glioblastoma lines, U87MG (0.015 nM) and U118MG (0.02 nM). A similar enhancement was observed against the lung carcinoma cell line, Calu-3 (0.0018 nM). Enhanced activity could not be explained by an increased number of cytokines available for binding since a combination of monospecific DTEGF and DTIL13 did not cause the same enhanced activity. Enhanced activity was dependent on the presence of both cytokines on the same single chain molecule and killing was receptor specific since target receptor negative leukemia cells were unaffected by the highly selective DTEGF13 and cytotoxicity could be blocked with anti-EGFR and anti-IL-13 antibodies. In a xenograft flank tumor model, intratumoral injection of DTEGF13, but not monospecific DTEGF or DTIL13, significantly inhibited the growth of established U87 tumors in nude mice (P < 0.04). In this model, the human EGF and IL-13 components of DTEGF13 are reactive with mouse EGFR and IL-13R, respectively. These studies show that a new co-targeting agent that simultaneously recognizes EGFR and IL-13R is more effective than its monospecific counterparts and that DTEGF13 has therapeutic advantages for glioblastoma.

IL-13 induces disease-promoting type 2 cytokines, alternatively activated macrophages and allergic inflammation during pulmonary infection of mice with Cryptococcus neoformans

In the murine model of Cryptococcus neoformans infection Th1 (IL-12/IFN-gamma) and Th17 (IL-23/IL-17) responses are associated with protection, whereas an IL-4-dependent Th2 response exacerbates disease. To investigate the role of the Th2 cytokine IL-13 during pulmonary infection with C. neoformans, IL-13-overexpressing transgenic (IL-13Tg(+)), IL-13-deficient (IL-13(-/-)), and wild-type (WT) mice were infected intranasally. Susceptibility to C. neoformans infection was found when IL-13 was induced in WT mice or overproduced in IL-13Tg(+) mice. Infected IL-13Tg(+) mice had a reduced survival time and higher pulmonary fungal load as compared with WT mice. In contrast, infected IL-13(-/-) mice were resistant and 89% of these mice survived the entire period of the experiment. Ag-specific production of IL-13 by susceptible WT and IL-13Tg(+) mice was associated with a significant type 2 cytokine shift but only minor changes in IFN-gamma production. Consistent with enhanced type 2 cytokine production, high levels of serum IgE and low ratios of serum IgG2a/IgG1 were detected in susceptible WT and IL-13Tg(+) mice. Interestingly, expression of IL-13 by susceptible WT and IL-13Tg(+) mice was associated with reduced IL-17 production. IL-13 was found to induce formation of alternatively activated macrophages expressing arginase-1, macrophage mannose receptor (CD206), and YM1. In addition, IL-13 production led to lung eosinophilia, goblet cell metaplasia and elevated mucus production, and enhanced airway hyperreactivity. This indicates that IL-13 contributes to fatal allergic inflammation during C. neoformans infection.

Mechanistic analysis of experimental food allergen-induced cutaneous reactions

Individuals with food allergy often present with uritcaria and atopic dermatitis. Indeed, susceptibility to food allergy may predispose to the development of these cutaneous allergic disorders. Recently, we developed a model of food allergy, whereby oral consumption of food [pea Pisum sativum L.; expressing alpha-amylase inhibitor-1 (alphaAI) from the common bean Phaseolus vulgaris L. cv Tendergreen (pea-alphaAI)] promotes a T helper cell type 2 (Th2) inflammatory response and predisposes to cutaneous allergic reactions following subsequent food allergen (alphaAI) exposure. To delineate the kinetics of food allergen-induced cutaneous reactions and examine the inflammatory mechanisms involved in this allergic reaction, we used interleukin (IL)-13-, IL-4 receptor alpha-, and eotaxin-1-deficient mice and performed serum transfer and CD4+ T cell depletion studies. We demonstrate that consumption of pea-alphaAI promotes an alphaAI-specific immunoglobulin G1 (IgG1) and IgE antibody response. Furthermore, we show that subsequent food allergen (alphaAI) challenge in the skin induced an early (3 h)- and late-phase (24 h) cutaneous allergic reaction. The early-phase response was associated with mast cell degranulation and the presence of Ig, whereas the late-phase response was characterized by a lymphoid and eosinophilic infiltrate, which was critically regulated by CD4+ T cells, IL-13, and eotaxin-1. Collectively, these studies demonstrate that food allergy can predispose to cutaneous inflammatory reactions, and these processes are critically regulated by Th2 immune factors.

Low expression of the interleukin (IL)-4 receptor alpha chain and reduced signalling via the IL-4 receptor complex in human neonatal B cells

Diminished neonatal antibody responses following infection or immunization may stem in part from intrinsic characteristics of neonatal B cells. In this study, we used B-cell subset sorting combined with gene expression assays to investigate major differences in the expression of host genes in neonatal and adult naïve B cells. We discovered significantly reduced expression of the interleukin (IL)-4 receptor alpha chain and reduced IL-4-induced signalling in neonatal B cells. Neonatal naïve B cells were susceptible to more rapid and more profound levels of apoptosis when cultured in vitro. They also exhibited a limited response to IL-4 treatment compared with adult cells. The expression level of the IL-13 receptor alpha 1 chain, a key component of the IL-13 receptor/IL-4 type II receptor, and the response to IL-13 treatment for protection against apoptosis in neonatal B cells were similar to those of the adult B cells. These studies suggest a possible mechanism underlying the limited magnitude and durability of neonatal antibody responses.

IL-13 transgene state impairs mycobacterial (type-1) and schistosomal (type-2) antigen-elicited responses

Transgenic technology provides one approach for examining cytokine properties in vivo. This study directly tested the effect of a lung-targeted IL-13 transgene on the induction and elicitation of Th1 and Th2 cell-mediated immuno-inflammatory responses. Induction of Th1 (type 1) and Th2 (type 2) responses were tested by sensitization of IL-13 transgenics and littermates with purified protein derivative (PPD) of Mycobacterium bovis or Schistosoma mansoni eggs. Secondary elicitation of pulmonary granulomas was examined in adoptively sensitized transgenics and littermates challenged with bead-bound PPD or S. mansoni egg antigens. Parameters included lymphoid tissue cytokine profiles and granuloma sizes. Results showed that induction and elicitation of both type 1 and type 2 cytokines and granulomas were significantly abrogated in transgenics. Systemic effects were possible, as transgenic serum contained high levels of circulating IL-13. These findings support the concept that IL-13 impairs effector functions and provide novel information regarding its role in regulating Th2 cytokines.

Interleukin 4 and 13 participation in mycobacterial (type-1) and schistosomal (type-2) antigen-elicited pulmonary granuloma formation: multiparameter analysis of cellular recruitment, chemokine expression and cytokine networks

The contribution of IL-4 and IL-13 to inflammation and cytokine responses was compared in mice with types-1 or -2 pulmonary granulomas (GR) elicited by beads bound to antigens of Mycobacteria bovis (PPD) or Schistosoma mansoni eggs (SEA). Type-2 SEA-GR produced the most IL-4 and IL-13. Type-1 PPD-GR produced detectable IL-13, but not IL-4. Mice were treated with anti-IL4 or anti-IL-13 Abs, then lesion size/composition, cytokine/chemokine mRNA and lymph node cytokines were measured. Type-1 GRs resisted individual Abs, but combined Abs augmented lesions by 20%. In contrast, anti-IL-4 abrogated type-2 GR by 30-40% and eosinophil recruitment by 60%. Anti-IL-13 abrogated type-2 GR by 20-30% with no effect on eosinophils. Combined depletion reduced lesion area by 60% and eosinophils by more than 80%. In type-1 GR lungs, anti-IL-4 and anti-IL-13 augmented IFNgamma and TNFalpha mRNA. In type 2 lungs, anti-IL-13 did likewise, but anti-IL-4 decreased TNFalpha without affecting IFNgamma mRNA. In both responses, IL-4 promoted MCP-1 and MCP-5 mRNA, but IL-13 inhibited chemokines in type-1 GR. In lymph nodes, anti-IL-4, but not anti-IL-13, abrogated type-2 cytokines. In fact, IL-13 down-regulated itself and other type-2 cytokines. In summary, IL-4 and IL-13 have common and disparate regulatory functions in types 1 and 2 responses.

Regulation of 15-lipoxygenase expression and mucus secretion by IL-4 in human bronchial epithelial cells

Our laboratory has recently shown that mucus differentiation of cultured normal human tracheobronchial epithelial (NHTBE) cells is accompanied by the increased expression of 15-lipoxygenase (15-LO). We used differentiated NHTBE cells to investigate the regulation of 15-LO expression and mucus secretion by inflammatory cytokines. Interleukin (IL)-4 and IL-13 dramatically enhanced the expression of 15-LO, whereas tumor necrosis factor-alpha, IL-1beta, and interferon (IFN)-gamma had no effect. These cytokines did not increase the expression of cyclooxygenase-2, with the exception of a modest induction by IL-1beta. The IL-4-induced 15-LO expression was concentration dependent, and mRNA and protein expression increased within 3 and 6 h, respectively, after IL-4 treatment. In metabolism studies with intact cells, 15-hydroxyeicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) were the major metabolites formed from exogenous arachidonic acid and linoleic acid. No prostaglandins were detected. IL-4 treatment dramatically increased the formation of 13-HODE and 15-HETE compared with that in untreated NHTBE cells, and several additional 15-LO metabolites were observed. Pretreatment of NHTBE cells with IFN-gamma or dexamethasone did not inhibit the IL-4-induced expression of 15-LO except at high concentrations (100 ng/ml of IFN-gamma and 10 microM dexamethasone). IL-4 treatment inhibited mucus secretion and attenuated the expression of the mucin genes MUC5AC and MUC5B at 12-24 h after treatment. Addition of 15-HETE precursor and 13-HODE precursor to the cultures did not alter mucin secretion or mucin gene expression. On the basis of the data presented, we conclude that the increase in 15-LO expression by IL-4 and attenuation of mucus secretion may be independent biological events.

Differential regulation of antigen-induced IL-4 and IL-13 generation from T lymphocytes by IFN-alpha

BACKGROUND

IL-4 and IL-13 are related cytokines with similar functional properties. Differential regulation of IL-4 and IL-13 has not been described.

OBJECTIVE

We have examined the effects of IFN-alpha on antigen-driven proliferation, IL-4 generation, and IL-13 generation from human PBMCs and T-cell clones.

METHODS

Proliferation was assessed by 3H-thymidine incorporation. Cytokine generation was assessed by reverse transcription PCR and ELISA. Messenger RNA stability was assessed in the presence of actinomycin D.

RESULTS

IFN-alpha induced a concentration-dependent inhibition of antigen-driven proliferation of TH1 and TH2 clones (median effective concentration, 150 to 200 U/mL); the sensitivity of TH1 and TH2 clones to IFN-alpha was not significantly different (P =.6). IFN-alpha induced an analogous concentration-dependent inhibition of antigen-driven IL-13 generation from TH1 and TH2 clones (median effective concentration, 100 U/mL); this effect was evident by 12 hours of culture and persisted beyond 48 hours. However, IL-4 generation from TH2 clones was insensitive to IFN-alpha at all concentrations and times tested (1 to 10,000 U/mL). A similar inhibitory effect of IFN-alpha on mitogen-driven proliferation and IL-13 generation from PBMCs was demonstrated; once again, IL-4 generation from PBMCs was insensitive to IFN-alpha. IL-13 mRNA stability was unaffected by IFN-alpha, suggesting transcriptional regulation.

CONCLUSION

IFN-alpha differentially regulates antigen-stimulated IL-4 and IL-13 generation.

First-trimester human chorionic villi express both immunoregulatory and inflammatory cytokines: a role for interleukin-10 in regulating the cytokine network of pregnancy

PROBLEM

T-helper 2 (TH2)-type cytokines [i.e., interleukin (IL)-6, IL-10, and IL-13] and transforming growth factor (TGF)-beta are expressed by the murine decidua and/or placenta and are likely to suppress inflammatory cytokine [i.e., IL-2, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, IL-1 alpha, and IL-1 beta] production at the maternal-fetal interface. In addition, class I IFNs may protect the fetus from immunologic rejection and viral infections. This study examines the expression of inflammatory/immunoregulatory cytokines and IL-10 production by first-trimester chorionic villi.

METHOD OF STUDY

Gestational tissues (n = 5) were obtained following elective terminations performed between 7 and 9 weeks of gestation. Chorionic villous tissues were separated from fetal membranes and decidua, and total RNA was extracted. Cytokine expression was assessed by a reverse transcriptase-polymerase chain reaction technique. Chorionic villi (n = 9; 6-12 weeks gestation) were maintained in organ culture, and human chorionic gonadotropin (hCG) and IL-10 levels were determined by immunoradiometric and enzyme-linked immunosorbent assays, respectively.

RESULTS

IFN-gamma and IL-2 were generally not expressed by first-trimester chorionic villi. Low to moderate levels of expression were noted for IL-1 alpha, IL-1 beta, and TNF-alpha. High levels of mRNA were noted for IFN-alpha and IFN-beta, but IFN-tau was not expressed. In all tissues, TGF-beta 1 and IL-13 were either weakly expressed or not expressed. In contrast, moderate to high levels of IL-6 and IL-10 mRNA were detected in each chorionic villous sample. In chorionic villous explants obtained at 6-11 weeks gestation production of hCG and IL-10 was greatest during the first 24 hr ([hCG] = 6961 +/- 815 mIU/mL, [IL-10] = 92 +/- 11 pg/mL) and then declined through 72 hr.

CONCLUSIONS

TH1-type cytokines (IL-2, IFN-gamma) are not expressed by first-trimester chorionic villous tissues: This is possibly due to local production of IL-10. In contrast, macrophage-associated cytokines (IL-1 beta and TNF-alpha) are expressed and their regulation may be critical for fetal survival. Finally, class 1 IFNs expressed by early chorionic tissues may protect the fetus from maternal rejection and viral transmission.

Interleukin-13 secretion by normal and posttransplant T lymphocytes; in vitro studies of cellular immune responses in the presence of acute leukaemia blast cells

T lymphocyte secretion of interleukin-13 (IL-13) in response to different activation signals was characterized in vitro. IL-13 release was investigated when virus transformed B lymphocytes or acute myelogenous leukaemia (AML) blasts were used as accessory cells during T cell activation. First, a majority of both CD4+ and CD8+ TCR alpha beta + T lymphocyte clones, derived from normal individuals and bone marrow transplant recipients, secreted IL-13 in response to a standardized mitogenic activation signal (phytohaemagglutinin + IL-2 + B lymphocyte accessory cells). The CD4+ cells showed significantly higher IL-13 levels than the CD8+ subsets. Second, when leukaemic accessory cells (more than 95% AML blasts) were used during T cell activation, IL-13 was released both during alloactivation of normal T lymphocytes and during mitogen activation of posttransplant T cells. Third, when normal T lymphocytes were stimulated with allogeneic AML blasts, addition of IL-13-neutralizing monoclonal antibodies decreased interferon gamma levels. Although addition of IL-13-neutralizing antibodies did not alter granulocyte-colony-stimulating factor secretion by allostimulating AML blasts, altered blast proliferation was detected for certain patients. Thus, most T cell clones can release IL-13, and IL-13 can modulate cytokine responses during T cell recognition of allogeneic AML cells.

Host factors in the pathogenesis of HIV disease

Host factors play an important role in determining rates of disease progression in human immunodeficiency virus (HIV)-infected individuals. HIV is able to subvert the host immune system by infecting CD4+ T cells that normally orchestrate immune responses and by inducing the secretion of proinflammatory cytokines that the virus can utilize to its own replicative advantage. The recognition that certain chemokine receptors serve as necessary co-factors for HIV entry into its target cells as well as the fact that ligands for these receptors can modulate the efficiency of HIV infection has expanded the number and scope of host factors that may impact the pathogenesis of HIV disease. This area of investigation will no doubt yield novel therapeutic strategies for intervention in HIV disease; however, caution is warranted in light of the enormous complexity of the pleiotropic cytokine and chemokine networks and the uncertainty inherent in manipulating these systems. HIV-infected long-term non-progressors represent an excellent model to study potential host factors involved in HIV disease pathogenesis. Genetic factors certainly have a major impact on the immune responses mounted by the host. In this regard, a polymorphism in the gene for the HIV co-receptor CC chemokine receptor 5 (CCR5), which serves as a co-receptor for macrophage (M)-tropic strains of HIV, affords a high degree of protection against HIV infection in individuals homozygous for the genetic defect and some degree of protection against disease progression in HIV-infected heterozygotes. HIV-specific immune responses, including cytotoxic T-lymphocyte (CTL) responses and neutralizing antibody responses, also appear to play salutary roles in protecting against disease progression.