The role of IL­16 gene polymorphisms in endometriosis.

Endometriosis is one of the most common gynecological diseases affecting up to 10% of the female population of childbearing age and a major cause of pain and infertility. It is influenced by multiple genetic, epigenetic and environmental factors. Interleukin­16 (IL­16) is a proinflammatory cytokine playing a pivotal role in many inflammatory and autoimmune diseases as well as in the pathogenesis of endometriosis. The aim of the present study was to evaluate the association of two IL­16 gene single nucleotide polymorphisms (SNPs), rs4072111 and rs11556218, with the risk of endometriosis in women from Greece as well as to gain insight about the structural consequences of these two exonic SNPs regarding development of the disease. A total of 159 women with endometriosis (stages I­IV) hospitalized for endometriosis, diagnosed by laparoscopic intervention and histologically confirmed, and 146 normal controls were recruited and genotyped. Subjects were genotyped using a polymerase chain reaction restriction fragment length polymorphism (PCR­RFLP) strategy. A significant association was detected regarding the GG and GT genotype as well as 'G' allele of rs11556218 in patients with endometriosis. The rs4072111 SNP of the IL­16 gene was not found to be associated with an increased susceptibility to endometriosis either for all patients (stages I­IV) or for stage III and IV of the disease only. Our results demonstrated that rs11556218 is associated with endometriosis in Greek women, probably by resulting in the aberrant expression of IL­16, as suggested by the bioinformatics analysis conducted on the SNP­derived protein sequences, which indicated a possible association between mutation and functional modification of Pro­IL­16.

Molecular characterization and expression analysis of large yellow croaker (Larimichthys crocea) interleukin-12A, 16 and 34 after poly I:C and Vibrio anguillarum challenge.

Interleukin-12, 16 and 34 are important pro-inflammatory cytokines, some of the most important components of the innate immunity system. Herein, we identified interleukin-12A (lcIL12A), 16 (lcIL16) and 34 (lcIL34) in large yellow croaker (Larimichthys crocea), and determined their expression profile in unchallenged and challenged tissues. The coding sequence (CDS) of lcIL12A comprised 600 bp long encoding a protein of 199 amino acids (aa), the CDS of lcIL16 was 2454 bp encoding a protein of 817 aa, and the CDS of lcIL34 was 657 bp encoding a protein of 267 aa. Phylogenetic analysis revealed similar results to homology comparison that lcIL12A was closest to IL12A of Dicentrarchus labrax (73%) and Serola dumerili (73%), while lcIL16 had the closest relation to Lates calcarofer (72.6%), and lcIL34 to Sparus aurata (88.9%). Multiple sequence alignment showed these interleukins were highly conserved with other vertebrate interleukins in their functional domains. Further, quantitative real time PCR (qPCR) analysis revealed that lcIL12A, lcIL16 and lcIL34 were constitutively expressed in all examined tissues, with significantly higher expression in spleen, liver and kidney. This was especially true for lcIL34 gene. Importantly, when challenged with polyinosinic:polycytidylic acid (poly I:C) and Vibrio anguillarum (V. anguillarum), the mRNA expressions of these interleukins were up-regulated in liver, spleen and kidney. Their top values got over 4 folds at least relative to their expression at time 0, and even lcIL12 reached 13.37 fold at 12-h point in spleen. These suggested their anti-viral and anti-bacterial roles and their involvement in the innate immune response of Larimichthys crocea. These results would have major implications in improving our understanding of the functions of interleukins to defend against pathogen infections in teleost species.

Identification and characterization of pro-interleukin-16 from mud crab Scylla paramamosain: The first evidence of proinflammatory cytokine in crab species.

IL-16 is a pro-inflammatory cytokine originally designated as a lymphocyte chemoattractant factor. In mammal and avian, it has been characterized as an essential regulator of various cellular processes including cell recruitment and activation against pathogen invasion. So far, neither of the full-length of IL-16 homologue nor the response mechanism against pathogen was reported in crab species. In the present study, the pro-IL-16 homologue was firstly cloned and characterized from mud crab Scylla paramamosain. The full-length Sp-pro-IL-16 consisted of 4107 bp with an opening reading frame encoding 1369 amino acids. Multiple alignment analysis showed the putative amino acid sequence of Sp-pro-IL-16 had about 73.86% identity with Litopenaeus vannamei pro-IL-16. Additionally, two conserved PDZ domains and protein binding sites were found in Sp-pro-IL-16 and showed high similarities about 94.19% and 51.14% with their Litopenaeus vannamei and Mus musculus counterparts. RT-PCR analysis indicated that Sp-pro-IL-16 transcripts were constitutively expressed in all tissues examined with an extreme high level in hepatopancreas. Moreover, Sp-pro-IL-16 transcripts in hepatopancreas were significantly up-regulated 15-fold at 72 h after Vibrio alginolyticus challenge and 3.5-fold at 12 h after virus-analog Poly (I:C) challenge. The Western blot analysis revealed that Sp-pro-IL-16 can be cleaved to its bioactive form, an approximately 35 kDa mature IL-16, and the protein levels of both pro-IL-16 and mature IL-16 increased after Vibrio alginolyticus challenge. It is the first experimental identification of pro-inflammatory cytokine IL-16 in arthropods. This study could shed new light on further understanding of the response mechanism of pro-inflammatory cytokine IL-16 in Scylla paramamosain against pathogens. Meanwhile, it brought new insight into the origin and evolution of IL-16 in crab species.

Structure of a Potential Therapeutic Antibody Bound to Interleukin-16 (IL-16): MECHANISTIC INSIGHTS AND NEW THERAPEUTIC OPPORTUNITIES.

Interleukin-16 (IL-16) is reported to be a chemoattractant cytokine and modulator of T-cell activation, and has been proposed as a ligand for the co-receptor CD4. The secreted active form of IL-16 has been detected at sites of TH1-mediated inflammation, such as those seen in autoimmune diseases, ischemic reperfusion injury (IRI), and tissue transplant rejection. Neutralization of IL-16 recruitment to its receptor, using an anti-IL16 antibody, has been shown to significantly attenuate inflammation and disease pathology in IRI, as well as in some autoimmune diseases. The 14.1 antibody is a monoclonal anti-IL-16 antibody, which when incubated with CD4(+) cells is reported to cause a reduction in the TH1-type inflammatory response. Secreted IL-16 contains a characteristic PDZ domain. PDZ domains are typically characterized by a defined globular structure, along with a peptide-binding site located in a groove between the αB and ßB structural elements and a highly conserved carboxylate-binding loop. In contrast to other reported PDZ domains, the solution structure previously reported for IL-16 reveals a tryptophan residue obscuring the recognition groove. We have solved the structure of the 14.1Fab fragment in complex with IL-16, revealing that binding of the antibody requires a conformational change in the IL-16 PDZ domain. This involves the rotation of the αB-helix, accompanied movement of the peptide groove obscuring tryptophan residue, and consequent opening up of the binding site for interaction. Our study reveals a surprising mechanism of action for the antibody and identifies new opportunities for the development of IL-16-targeted therapeutics, including small molecules that mimic the interaction of the antibody.

Genetic variants in microRNA target sites of 37 selected cancer-related genes and the risk of cervical cancer.

OBJECTIVES: Single nucleotide polymorphisms (SNPs) in putative microRNA binding sites (miRSNPs) modulate cancer susceptibility via affecting miRNA binding. Here, we sought to investigate the association between miRSNPs and cervical cancer risk. METHODS: We first genotyped 41 miRSNPs of 37 cancer-related genes in 338 patients and 334 controls (Study 1), and replicated the significant associations in 502 patients and 600 controls (Study 2). We tested the effects of miRSNPs on microRNA-mRNA interaction by luciferase reporter assay. RESULTS: Five SNPs displayed notable association with cervical cancer risk in Study 1. Only IL-16 rs1131445 maintained a significant association with cervical cancer (CT/CC vs. TT, adjusted OR = 1.51, P = 0.001) in Study 2. This association was more evident in the combined data of two studies (adjusted OR = 1.49, P = 0.00007). We also found that miR-135b mimics interacted with IL-16 3'-UTR to reduce gene expression and that the rs1131445 T to C substitution within the putative binding site impaired the interaction of miR-135b with IL-16 3'-UTR. An ELISA indicated that the serum IL-16 of patients with cervical cancer was elevated (vs. controls, P = 0.001) and correlated with the rs1131445 genotype. Patients who carried the rs1131445 C allele had higher serum IL-16 than non-carriers (P<0.001). CONCLUSIONS: These results support our hypothesis that miRSNPs constitute a susceptibility factor for cervical cancers. rs1131445 affects IL-16 expression by interfering with the suppressive function of miR135b and this variant is significantly associated with cervical cancer risk.

lnterleukin-16: the ins and outs of regulating T-cell activation.

Intcrleukin-16 (IL-16) was originally identified in 1980 as the first-described T-cell chemoattractant. Since that time, the protein has been cloned, sequenced, and characterized in terms of expression and biologic function for regulating inflammation. Generated as a precursor molecule, IL-16 is cleaved by caspase-3, yielding pro-IL-16 and the secreted (mature) portion. Both components of IL-16 are now known to regulate T-cell growth and represent one of the few proteins for which function has been determined for both the pro- and secreted portions. Secreted IL-16 primes CD4+ T cells for IL-2 and IL-15 responsiveness, with a preferential effect on TH1 cells. Animal models have identified its involvement in the establishment of TH1-type inflammation with a critical role in the development of certain autoimmune diseases. Nuclear pro-IL-16 is a recently identified regulator of Skp2 transcription and T- cell cycle progression, acting as a scaffold protein for GABPbeta and histone deacctylase-3. The intent of this review is to present an update on the structure of both IL-16 and pro-IL-16, the biologic functions of both components, and how the functions relate to the pathology of certain diseases where changes in IL-16 expression levels have been detected.

Not-so-sweet sixteen: the role of IL-16 in infectious and immune-mediated inflammatory diseases.

Over the past two decades, our understanding of interleukin-16 (IL-16) has increased substantially. Initial studies characterizing IL-16 as a chemotactic cytokine (but not a chemokine) just scratched the surface of the unique properties of this cytokine. Since then, scientists have determined that IL-16 has a wide range of effects on cells, including upregulation of CD25, induction of cells to progress to the G(1) phase, inhibition of antigen- specific proliferation yet with retained antigen nonspecific proliferative properties, and discovery of a novel neuronal form with unique properties. Recently, a plethora of studies have implicated IL-16 in exacerbation of infectious, immune-mediated, and autoimmune inflammatory disorders, including atopic dermatitis, irritable bowel syndrome, systemic lupus erythematosus, neurodegenerative disorders, and viral infections. Herein, we review the body of evidence supporting a role for IL-16 in infectious and immune-mediated inflammatory disorders and explore the known and possible mechanism of actions in the numerous diseases.

Identification of pro-interleukin 16 as a novel target of MAP kinases in activated T lymphocytes.

T lymphocyte activation is controlled by a coordinated web of tyrosine and serine kinases. There is a large body of information about tyrosine kinase substrates in T cells but analysis of serine kinase substrates has been more difficult. Recently we described an antiserum that recognizes serine-phosphorylated peptides corresponding to the substrate sequences for AGC serine kinases. This antiserum, termed PAP-1 (phospho antibody for proteomics-1), has proven useful for probing the serine phosphoproteome of antigen receptor-activated T lymphocytes. The present study shows that PAP-1 can also be used to explore serine kinases activated by cytokines and chemokines in T cells. Using PAP-1, together with proteomic analysis, the precursor form of the cytokine IL-16 (ProIL-16) was shown to be phosphorylated on Ser144 in antigen receptor-, SDF1alpha- and IL-2-activated T cells. Genetic and pharmacological-inhibitor experiments showed that the phosphorylation of ProIL-16 is dependent on activation of the kinases Erk1/2. IL-16 is secreted by mitogen-activated T cells, and the biochemical link between ProIL-16 and Erk1/2, revealed by studies with PAP-1, prompted analysis of the role of MAP kinases in this response. We show that TCR-mediated secretion of IL-16 is dependent on MAP kinases. The present study thus reveals how phosphoproteomic analysis opens previously unrecognized avenues for research, and yields novel insights about targets for MAP kinases in T lymphocytes.

PDZ Domain-mediated interaction of interleukin-16 precursor proteins with myosin phosphatase targeting subunits.

The cytokine interleukin-16 is generated by posttranscriptional cleavage by caspase-3 of two large precursor isoforms. The smaller protein of 67 kDa (pro-IL-16) is expressed in cells of the immune system and contains three PDZ (postsynaptic density/disc large/zona occludens-1) domains, whereas the larger 141-kDa neuronal variant (npro-IL-16) has two additional PDZ domains in its N-terminal extension that interact with neuronal ion channels. Using the yeast two-hybrid approach we have identified three closely related myosin phosphatase targeting subunits, MYPT1, MYPT2, and MBS85, as binding partners of the IL-16 precursor proteins. These interactions were verified using pull-down assays, coimmunoprecipitations, and plasmon resonance experiments. Binding requires the intact PDZ2 domain of pro-IL-16 and highly related C-terminal regions in the ligands consisting of a short leucine zipper and an indispensable serine at the -1 position, suggesting a novel unconventional PDZ binding mode. Pro-IL-16 and the myosin phosphatase targeting subunits colocalize along actomyosin filaments and stress fibers in transfected COS-7 cells. By modulating and targeting the catalytic phosphatase subunit to its substrates, MYPT1, MYPT2, and MBS85 regulate various contractile processes in muscle and non-muscle cells. Our findings indicate an involvement of the IL-16 precursor molecules in myosin-based contractile processes, most likely in cell motility, providing a functional link to the chemotactic activity of the mature cytokine. Alternatively, an intracellular complex of npro-IL-16, ion channels, and components of myosin motors in neurons suggests a role in protein targeting.

Binding of HTLV-1 tax oncoprotein to the precursor of interleukin-16, a T cell PDZ domain-containing protein.

HTLV-1 Tax oncoprotein interacts with various cellular factors and modulates transcription and the cell cycle. In that role it is sufficient to create T cell malignancies in the absence of HTLV-1 infection. HTLV-1 Tax protein has been reported to bind to cellular proteins containing PDZ domains in vitro. The precursor of human interleukin 16, pro-IL-16, is an abundant cellular protein present in human peripheral blood T cells. Pro-IL-16 contains three PDZ domains. It has been shown that expression of pro-IL-16 in pro-IL-16 negative cells induces a G(0)/G(1) arrest in the cell cycle. The current studies demonstrate that Tax binds to pro-IL-16 in HTLV-1 infected human T cells. We mapped the Tax binding site to the first PDZ domain of pro-IL-16. Over-expression of Tax in COS cells resulted in fewer cells in G(0)/G(1) consistent with its activity to induce G(1)- to S-phase progression in lymphocytes, while over-expression of pro-IL-16 in COS cells resulted in G(0)/G(1) arrest. Co-expression of wild type Tax with pro-IL-16 in COS cells negated the effects of pro-IL-16, an effect not observed with Tax mutated at its PDZ binding C-terminus. These results suggest that one of the effects of Tax on growth deregulation in HTLV-1 infected T cells might be mediated by its binding to pro-IL-16.

Nuclear translocation of the N-terminal prodomain of interleukin-16.

Interleukin-16 (IL-16) is a pleiotropic cytokine that functions as a chemoattractant factor, a modulator of T cell activation, and an inhibitor of human immunodeficiency virus (HIV) replication. These diverse functions are exclusively attributed to the secreted C-terminal peptide of 121 amino acids (mature IL-16), which is cleaved from the precursor protein (pro-IL-16) by caspase-3. Human pro-IL-16 is comprised of 631 amino acids with three PDZ domains, one of which is present in secreted mature IL-16. No cellular localization or biologic functions have been ascribed to the unusually large and highly conserved N-terminal prodomain formed as a result of proteolytic release of the third PDZ domain of pro-IL-16. Here we show that the N-terminal prodomain of pro-IL-16 translocates into the nucleus following cleavage of the C-terminal segment. The nuclear localization signal of pro-IL-16 consists of a classical bipartite nuclear targeting motif. We also show that the nuclear targeting of the IL-16 prodomain induces a G(0)/G(1) arrest in the cell cycle. Taken together, the high degree of conservation of the prodomain among species, the presence of two PDZ motifs, and the nuclear localization and subsequent inhibitory effect on cell cycle progression suggest that pro-IL-16 is cleaved into two functional proteins, a C-terminal-secreted cytokine and an N-terminal product, which affects the cell cycle.

Interleukin-16.

Interleukin 16 (IL-16) was initially described in 1982 as the first T cell chemoattractant. Through interaction with CD4, IL-16 has now been characterized as a chemoattractant for a variety of CD4+ immune cells. Recent in vivo studies have more fully characterized IL-16 as an immunomodulatory cytokine that contributes to the regulatory process of CD4+ cell recruitment and activation at sites of inflammation in association with asthma and several autoimmune diseases. Since its cloning in 1994, IL-16 structure and function have been studied extensively. This review addresses the current data regarding IL-16 protein and gene structure; the expanding list of cells capable of generating IL-16; the direct interaction of IL-16 with its receptor, CD4; and the functional bioactivities of IL-16 as they relate to inflammation and HIV-1 infection. In addition, potential therapeutic modalities for IL-16 relating to inflammation and immune reconstitution in HIV-1 infection are also discussed.

Interleukin 16: implications for CD4 functions and HIV-1 progression.

In this article, David Center and colleagues clarify the controversies that have emerged over the unique structure of interleukin 16 and its anti-HIV-1 activity. Interleukin 16 is a ligand for CD4, and this implies CD4 acts as a sentinel receptor that can switch CD4+ T cells between immune and inflammatory functions.

Interaction between human interleukin-16 and CD4 receptor of HIV-1.

AIM: To study the interaction between human interleukin-16 (IL-16) and the receptor CD4 (T-lymphocyte differentiation antigen) of human immunodeficiency virus type 1 (HIV-1). METHODS: Two structurally conserved regions (SCRs) of human IL-16 were built by the SYBYL/Biopolymer module using the corresponding transmembrane (TM) domain of human interleukin-4 (HIL-4) and HIL-2 as the templates. The coordinates for amino-terminal residue sequence, carboxyl-terminal residue sequences, and cytoplasm loops were generated using Biopolymer's LOOP SEARCH algorithm. RESULTS: HIL-16 first formed a homodimer, then contacted with CD4 dimer further forming a dimeric complex. Subsequently, the dimeric complex constructed the tetrameric complex by two disulfide bridges between the cysteines of HIL-16 (Cys31-Cys31). CONCLUSION: The interaction model is useful to propose the action mechanism of HIL-16 and is beneficial for rational designing of novel anti-HIV drugs.

Neuronal interleukin-16 (NIL-16): a dual function PDZ domain protein.

Interleukin (IL)-16 is a proinflammatory cytokine that has attracted widespread attention because of its ability to block HIV replication. We describe the identification and characterization of a large neuronal IL-16 precursor, NIL-16. The N-terminal half of NIL-16 constitutes a novel PDZ domain protein sequence, whereas the C terminus is identical with splenocyte-derived mouse pro-IL-16. IL-16 has been characterized only in the immune system, and the identification of NIL-16 marks a previously unsuspected connection between the immune and the nervous systems. NIL-16 is a cytosolic protein that is detected only in neurons of the cerebellum and the hippocampus. The N-terminal portion of NIL-16 interacts selectively with a variety of neuronal ion channels, which is similar to the function of many other PDZ domain proteins that serve as intracellular scaffolding proteins. Among the NIL-16-interacting proteins is the class C alpha1 subunit of a mouse brain calcium channel (mbC alpha1). The C terminus of NIL-16 can be processed by caspase-3, resulting in the release of secreted IL-16. Furthermore, in cultured cerebellar granule neurons undergoing apoptosis, NIL-16 proteolysis parallels caspase-3 activation. Cerebellar granule neurons express the IL-16 receptor CD4. Exposure of these cells to IL-16 induces expression of the immediate-early gene, c-fos, via a signaling pathway that involves tyrosine phosphorylation. This suggests that IL-16 provides an autocrine function in the brain. Therefore, we hypothesize that NIL-16 is a dual function protein in the nervous system that serves as a secreted signaling molecule as well as a scaffolding protein.

Identification of domains in IL-16 critical for biological activity.

IL-16 is a proinflammatory cytokine implicated in the pathogenesis of asthma and other conditions characterized by recruitment of CD4+ T cells to sites of disease. It is postulated that CD4 is an IL-16 receptor, although other receptors or coreceptors may exist. Among several known functions, IL-16 is a chemoattractant factor for CD4+ T cells and it inhibits MLR. We previously reported that an oligopeptide corresponding to the 16 C-terminal residues of human IL-16 inhibits chemoattractant activity. To identify functional domains with greater precision, shorter oligonucleotides containing native or mutated C-terminal IL-16 sequences were tested for IL-16 inhibition. Within the 16 C-terminal residues, the minimal peptide RRKS (corresponding to Arg106 to Ser109) was shown to mediate inhibition of IL-16 chemoattractant activity. Inhibition was lost when either arginine was substituted with alanine. Point mutations in IL-16 revealed that Arg107 is critical for chemoattractant activity, but MLR inhibition was unaffected by mutation of Arg107 or even deletion of the C-terminal tail through Arg106. Deletion of 12 or 22 N-terminal residues of IL-16 had no impact on chemoattractant activity, but MLR inhibition was reduced. Deletion of 16 C-terminal plus 12 N-terminal residues abolished both chemoattractant and MLR-inhibitory activity of IL-16. These data indicate that receptor interactions with IL-16 that activate T cell migration are not identical with those required for MLR inhibition, and suggest that both N-terminal and C-terminal domains in IL-16 participate in receptor binding or activation.

Molecular cloning and expression of feline interleukin-16.

Human IL-16 (hIL-16) is a homotetrameric cytokine with chemotactic properties towards cells expressing the CD4 receptor. This chemotactic cytokine plays an important role in attracting cells of the immune system to the site where CD8+ T-cells were activated for example by a foreign antigen. In addition to the chemotactic activity, hIL-16 also induces expression of IL-2 receptor, increasing the responsiveness to IL-2 and therefore implying a role for specific expansion of the CD4+ T-cell population in an area of induced inflammation. In this report we describe the cloning, sequencing and the expression of feline IL-16 (fIL-16). At the nucleotide level, fIL-16 shows 84.6 and 84.5%, on the amino acid level 93 and 91.5% identity to the human and African green monkey (agm) IL-16, respectively.

Structure of interleukin 16 resembles a PDZ domain with an occluded peptide binding site.

The structure of a folded core of IL-16 is similar to that of intracellular protein modules called PDZ domains. IL-16 is thus the first extracellular protein found to have a PDZ-like fold. However, it does not exhibit normal peptide binding properties of PDZ domains. This is due to alterations of the structure at the 'PDZ-like binding site' of IL-16 (the GLGF cleft): the GLGF cleft of IL-16 is much smaller than those of PDZ-domains and is additionally blocked with a tryptophan side chain at its center. Our experiments indicate also that IL-16 nonspecifically aggregates in solution; but formation of a homo-tetrameric protein is not required, in contrast to previous suggestions, for its chemo-attractant activity.

Conservation of structure and function between human and murine IL-16.

IL-16 is a proinflammatory cytokine that signals via CD4, inducing chemotactic and immunomodulatory responses of CD4+ lymphocytes, monocytes, and eosinophils. Comparative analysis of murine and human IL-16 homologs could reveal conserved structures that would help to identify key functional regions of these cytokines. To that end, we cloned the murine IL-16 cDNA and found a high degree of amino acid similarity comparing the predicted murine and human IL-16 precursor proteins (pro-IL-16). The highest similarity (82.1%) was found in the C-terminal region, which is cleaved from pro-IL-16 to yield biologically active IL-16. Chemotaxis experiments with IL-16 of murine and human origin, using murine splenocytes or human T lymphocytes as targets, showed cross-species stimulation of motility. Synthetic oligopeptides and anti-peptide Ab were produced, based on the sequences of three predicted hydrophilic domains of IL-16 potentially presented in exposed positions. None of these peptides had intrinsic IL-16 bioactivity, but one (corresponding to a hydrophilic C-terminal domain of IL-16) partially displaced binding of OKT4 mAb to human lymphocytes. This peptide, and its cognate Ab, also inhibited IL-16 chemoattractant activity for human and murine cells. These studies demonstrate a high degree of structural and functional similarity between human and murine IL-16 and suggest that amino acids in the C terminus are critical for its chemoattractant function. The data suggest cross-species conservation of IL-16 receptor structures as well. Inhibitory peptides may be useful in disease states where the proinflammatory functions of IL-16 are detrimental to the host.

Molecular cloning and sequence analysis of interleukin 16 from nonhuman primates and from the mouse.

Interleukin 16 (IL-16) is synthesized as a 67 000 Mr precursor (pro-IL-16), but only a carboxy terminal part of 12 000-14 000 Mr is secreted by CD8(+) lymphocytes. This lymphokine binds to CD4 and has been shown to induce migration, affect the activation state of T cells, and inhibit immunodeficiency virus replication. It has been suggested that CD8(+) cell-derived soluble factors play a pivotal role in protecting natural-host nonhuman primates from developing immunodeficiency following SIV infection. In a first attempt to address this question, we cloned and sequenced the IL-16 cDNA from different primates. Here we report the pro-IL-16 sequence from chimpanzees, African green monkeys (AGM), rhesus macaques, and cynomolgus macaques. In order to compare and analyze structural motifs possibly involved in processing, intracellular targeting, or secretion, we extended our study to the New World monkeys saimiri and aotus and to the mouse. Alignments of deduced amino acids reveal that the human protein shares 99% similarity to that of chimpanzees, approximately 95% to rhesus, cynomolgus and AGM, about 90% to aotus and saimiri, and 77.5% to the mouse. Phylogenetic analyses revealed the expected evolutionary groupings.