Perspectives IL-32: An Emerging Player in the Network against ? Manikuntala Kundu, Joyoti Basu*

nterleukin-32 (IL-32), which was Stimulation of IL-32 by Strengths and Weaknesses previously called natural killer Mycobacteria of the Study cell transcript 4, has recently been I Netea et al. explored the role of IL- The crucial role of IFN-γ in recognized as a proinfl ammatory 32 in the context of M. tuberculosis antimycobacterial immunity has (see Glossary) [1]. The main infection. In their study, freshly been shown in patients with defects sources of IL-32 are natural killer obtained human peripheral blood in the IFN-γ receptor [7]. Netea and cells, T cells, epithelial cells, and mononuclear cells (PBMCs) were colleagues’ fi ndings raise important blood monocytes. Four transcripts of stimulated with different TLR agonists, questions regarding (a) how IL-32 IL-32 are known at present. IL-32 has and gene expression and synthesis of levels vary in patients with defects in emerged as an important player in IL-32 was determined. The authors IFN-γ receptors who are susceptible innate and adaptive immune responses, showed that M. tuberculosis could elicit to tuberculosis and (b) how these and information is emerging on IL-32 release from PBMCs as well as variations may impact the course of synergism between IL-32 and other from purifed monocyte populations. the infection. One of the defi ciencies well-characterized players in innate of their study stems from the fact immunity. that the role of IL-32 in controlling The innate immune response M. tuberculosis-dependent cytokine depends on the recognition of IL-32 has emerged networks has not been investigated pathogen-associated molecular as an important player directly by neutralizing or blocking patterns by families of pathogen IL-32. Rather, the authors’ inferences recognition receptors. The best in innate and adaptive have been drawn indirectly through characterized among these are the immune responses. blocking IFN-γ and IL-18 signaling. Toll-like receptor (TLR) [2] and the PR3 has recently been identifi ed as nucleotide-binding oligomerization an IL-32 binding protein [8], and domain (NOD) [3] families of This is the fi rst documented instance therefore researchers can now explore proteins. Recent studies have shown of a probable role of IL-32 in the the effect of neutralizing IL-32 using that IL-32 synergizes with NOD1 and immune response elicited by an enzymatically inactive PR3 or PR3- NOD2 ligands to stimulate IL-1β and intracellular pathogen. The authors derived peptides. IL-6 release in a caspase-1-dependent also found that other heat-killed manner [4]. These fi ndings are organisms, such as Staphylococcus of potential clinical importance aureus , Candida albicans, or Aspergillus in settings where NOD2 plays a fumigatus, do not stimulate IL- Funding: The authors received no specifi c funding protective role, such as in Crohn’s 32 production, although these for this article. disease, where NOD2-dependent organisms are potent inducers of the Competing Interests: The authors declare that they production of defensins and proinfl ammatory cytokines IL-6 and have no competing interests. contributes to antimicrobial defense TNF-α. in the gut. These fi ndings are Citation: Kundu M, Basu J (2006) IL-32: An emerging This study showed that the player in the immune response network against also likely to be of signifi cance in expression of the genes of isoforms tuberculosis? PLoS Med 3(8): e274. DOI: 10.1371/ tuberculosis. Individuals homozygous α and γ of IL-32 is stimulated by M. journal.pmed.0030274 for the 3020ins C NOD2 mutation tuberculosis , whereas the gene for the DOI: 10.1371/journal.pmed.0030274 show a defective cytokine response to β isoform is constitutively expressed [5]. It is in (i.e., always expressed, even without Copyright: © 2006 Kundu and Basu. This is an tuberculosis open-access article distributed under the terms this context that the study reported by a stimulus). The authors provide of the Creative Commons Attribution License, Netea et al. in PLoS Medicine , in which evidence in favor of a role of IFN-γ in which permits unrestricted use, distribution, and the authors explored the regulation reproduction in any medium, provided the original the induction of IL-32 synthesis. They author and source are credited. of IL-32 production by primary cells went on to show that M. tuberculosis of the immune system, is of potential elicits caspase-1-dependent cleavage Abbreviations: IL-32, Interleukin-32; NOD, nucleotide-binding oligomerization domain; PBMC, importance [6]. of the precursor of IL-18, its release, peripheral blood mononuclear cell; TLR, toll-like and IL-18-dependent production of receptor; UTR, untranslated region IFN-γ, which could be blocked by IL- Manikuntula Kundu and Joyoti Basu are at the 18 blocking peptide. The authors also Department of Chemistry, Bose Institute, , The Perspectives section is for experts to discuss the claim that TNF-α release from PBMCs India. clinical practice or public health implications of a challenged with occurs in published article that is freely available online. M. tuberculosis * To whom correspondence should be addressed. an IL-32-independent manner. E-mail: [email protected]

PLoS Medicine | www.plosmedicine.org 1210 August 2006 | Volume 3 | Issue 8 | e274 In addition, the present report fails Glossary for autoimmune disorders, its role to address how the synthesis of the in mounting an effective immune Cytokines: Broad group of signaling different isoforms of the IL-32 protein response against invading pathogens proteins that are in general produced by is regulated, which will require future also needs to be considered. It immune cells after cell activation and act exploration. IL-32 has been reported to would be pertinent to bear in mind as autocrine or paracrine regulators of induce the pro-infl ammatory cytokines that neutralization of IL-32 could the immune response. TNF-α and IL-1β from murine render patients more susceptible to peritoneal , as well as from Natural killer cells: Large, granular, tuberculosis. The risk of tuberculosis phorbol ester-differentiated human bone-marrow-derived lymphocytes of in patients on anti-TNF-α therapy has THP-1 cells [1]. Netea et al. show that the innate immune system that release already been documented [13]. IL-32 production is not associated cytolytic molecules to kill infected cells The most important question raised with TNF-α induction. This raises the and tumor cells. by these studies is why M. tuberculosis question whether the differences in PAMPS: Specifi c conserved structures induces IL-32 whereas other organisms TNF-α induction are attributable to of pathogens recognized by pattern that are known inducers of IFN-γ intrinsic differences in behavior of recognition receptors on macrophages do not. The identifi cation of likely monocytic cells in comparison with and dendritic cells. additional IL-32 regulating pathways differentiated macrophages. Toll-like receptor: Germline-encoded triggered uniquely by mycobacteria The fi ndings reported in this study pattern recognition receptors that deserves immediate attention.  raise interesting questions and open recognize conserved molecular patterns References avenues for further exploration. shared by microorganisms. 1. Kim SH, Han SY, Azam T, Yoon DY, Dinarello One question is whether external C (2005) Interleukin-32: A cytokine and stimuli such as IFN-γ regulate IL-32 NOD families of proteins: A family inducer of TNFα. Immunity 22: 131–142. of cytoplasmic proteins that contain a 2. Beutler B, Jiang Z, Georgel G, Krozat K, at both the transcriptional as well Croker B, et al. (2006) Genetic analysis of host as the translational level. The likely nucleotide-binding site and a leucine- resistance: Toll-like receptor signalling and immunity at large. Annu Rev Immunol 24: roles of the 5′- and 3′-untranslated rich repeat and function as cytosolic sensors involved in innate recognition 353–389. regions (UTRs) of the IL-32 mRNA in 3. Inohara C, McDonald C, Nunez G (2005) regulating IL-32 production need to be of microorganisms and regulation of NOD-LRR proteins: Role in host-microbial infl ammatory responses. interactions and infl ammatory disease. Annu deciphered. IL-32 production has been Rev Biochem 74: 355–383. observed to be TLR independent but Interleukins: Secreted regulatory 4. Netea MG, Azam T, Ferwerda G, Giardin SE, proteins produced by immune cells Walsh M, et al. (2005) IL-32 synergizes with IFN-γ dependent [1]. MyD88, a classical nucleotide oligomerization domain (NOD) and TLR adapter protein [2] central to such as monocytes and lymphocytes in NOD2 ligands for IL-1β and IL-6 production TLR signaling, has recently been shown response to stimuli that help the immune through a caspase 1-dependent mechanism. Proc Natl Acad Sci 32: 16309–16314. to signal through the IFN-γ receptor system fi ght infection and diseases such 5. Ferwerda G, Girardin SE, Kulberg B, le Bourhis [9] through a non-Toll-IL-1R domain. as cancer. L, de Jong DJ, et al. (2005) NOD2 and Toll- MyD88 stabilizes mRNA through like receptors are nonredundant recognition Caspase-1 dependent: Refers to the systems of Mycobacterium tuberculosis . PLoS classical AU-rich elements found in thiol protease caspase-1-mediated Path 1: 279–285. DOI: 10.1371/journal. the 3′ UTRs of many mRNAs. Does proteolytic processing of the precursor ppat.0010034 6. Netea MG, Azam T, Lewis EC, Joosten LAB, MyD88 have any role in relation to the form of IL-18 to the mature, active form Wang M, et al. (2006) Mycobacterium tuberculosis production of IL-32? This question is of the cytokine. induces interleukin-32 production through particularly important in view of the a caspase-1/IL-18/interferon-γ dependent Defensins: Cationic, cysteine-rich mechanism. PLoS Med 3: In press. report showing fatal M. tuberculosis peptides found in the cytoplasmic 7. Doffi nger R, Dupuis S, Picard C, Fieschi C, infection in mice in the absence of Feinberg J, et al. (2002) Inherited disorders granules of neutrophils and macrophages MyD88 [10]. In general, interactions of IL-12- and IFNγ-mediated immunity: A possessing broad antimicrobial activity molecular genetics update. Mol Immunol 38: between 5′ and 3′ UTRs resulting 903–909. against bacteria, fungi, and enveloped in the formation of an RNA loop 8. Novick D, Rubinstein M, Azam T, Rabinov A, viruses. Dinarello CA, et al. (2006) Proteinase 3 is an increase translational effi ciency [11], IL-32 binding protein. Proc Natl Acad Sci 103: and RNA-binding proteins act at the PR3: A granule serine protease present 3316–3321. in neutrophils and monocytes capable 9. Sun D, Ding A (2006) MyD88-mediated level of enhancing or repressing these stabilization of interferon-γ-induced cytokine interactions to control translation. of processing a variety of biological and chemokine mRNA. Nat Immunol 7: 375– The control processes that regulate substrates. 381. 10. Fremond CM, Yeremeev V, Nicolle DM, translation of IL-32 are additional areas MyD88: An adapter molecule that binds Jacobs M, Quesniaux VF, et al. (2004) Fatal of future exploration. to the intracellular domains of TLRs and Mycobacterium tuberculosis infection despite recruits a number of molecules to the TLR adaptive immune response in the absence of MyD88. J Clin Invest 114: 1790–1799. Clinical Implications complexes to trigger signaling. 11. Mazumder B, Seshadri V, Fox PL (2003) For the clinician, IL-32 emerges as Translational control by the 3′-UTR: The ends specify the means. Trends Biochem Sci 28: yet another cytokine whose role in 91–98. the course of tuberculosis and related tuberculosis. IL-32 contributes to 12. Joosten LA, Netea MG, Kim SH, Yoon DY, infections deserves evaluation. The the synovitis of , Oppers-Walgreen B, et al. (2006) IL-32, a proinfl ammatory cytokine in rheumatoid cellular receptor (or interacting and the infl ammation of rheumatoid arthritis. Proc Natl Acad Sci 103: 3298–3303. partner) for IL-32 needs to be arthritis correlates with IL-32 gene 13. Mayordomo L, Marenco JL, Gomez-Mateos J, Rejon E (2002) Pulmonary miliary tuberculosis identifi ed in antigen-presenting cells, expression [12]. While considering in a patient with anti-TNF-alpha treatment. which are of particular relevance to intervention at the level of IL-32 Scand J Rheumatol 31: 44–45.

PLoS Medicine | www.plosmedicine.org 1211 August 2006 | Volume 3 | Issue 8 | e274