First printed in R&D Systems' 2000 Catalog.
Interleukin 16 (IL-16), formerly known as lymphocyte chemoattractant factor or LCF,1,2 is a pro-inflammatory cytokine that is chemotactic for CD4+ T lymphocytes, monocytes, and eosinophils. In addition to inducing chemotaxis, IL-16 can upregulate IL-2 receptor3 and HLA-DR4 expression, inhibit T cell receptor (TcR)/CD3-dependent activation,5,6 and promote repression of HIV-1 transcription.7 IL-16 is a unique cytokine with no significant sequence homology to other well-characterized cytokines or chemokines.
IL-16 was originally identified as a homotetramer consisting of individual 14 kDa monomers of 130 amino acids (aa) each.8 IL-16 is synthesized as a precursor molecule (pro-IL-16) of approximately 68 kDa and 631 aa lacking a signal peptide.9,10 The gene for IL-16 maps to chromosome 15 in the human11 and chromosome 7 in the mouse.12 The sequence and overall structure of IL-16 is conserved across species. Both human and mouse IL-16 genes comprise seven exons and six introns.12,13 Human and mouse IL-16 homologs display conservation of both structure and function, particularly in the C-terminal region (82.1% similarity).14 The human pro-IL-16 sequence displays >90% similarity to various non-human primates.15
Recombinant pro-IL-16 polypeptides are specifically cleaved in CD8+ cell lysates suggesting that the actual secreted form of IL-16 may be smaller than the originally published 130 aa form.9 One potential protease cleavage site within the C-terminal region of IL-16 occurs following an Asp residue implicating the involvement of a caspase family member. In CD8+ T cells, active caspase-3 cleaves pro-IL-16 producing a biologically active, secreted form of IL-16 (i.e., representing 121 C-terminal aa residues of pro-IL-16).16 The mechanism of release or secretion of IL-16, however, is currently unknown but does not appear to correlate with apoptosis.17
The aa sequence of mature, secreted IL-16 contains a PDZ motif. A PDZ motif consists of an approximately 90 aa residue unit, which is often repeated in a protein. They are typically intracellular protein modules involved in clustering ion channels, receptors, and other membrane proteins, connecting them to signaling complexes.18 A conserved GLGF (Gly-Leu-Gly-Phe) aa sequence characterizes PDZ motifs and functions in mediating binding of defined peptide sequences. The IL-16 PDZ motif does contain a “GLGF cleft”, however, it is much smaller than those of other PDZ-containing proteins and is blocked by a tryptophan side chain in its center.19 Blockage of the peptide-binding site prevents IL-16 from displaying expected peptide-binding properties of PDZ-like domains. The functional significance of the IL-16 PDZ motif remains to be identified.
CD4 serves as a signal-transducing receptor for IL-16. Expression of CD4 is required for mediating IL-16 functions.3,20-23 Interaction between IL-16 and CD4 can specifically initiate an increase in intracytoplasmic calcium and inositol trisphosphate,3 activation of p56lck,21 and translocation of protein kinase C from the cytosol to the cell membrane.23 IL-16-induced functions can be inhibited by a monomeric Fab of an anti-CD4 (OKT4) monoclonal antibody.4,8 The region of CD4 that binds IL-16 has been identified within the D4 domain, overlapping structure involved in CD4 dimer formation.24 Studies with point mutations or deletions in the C- and N-terminal residues of IL-16 demonstrate that sequences within both the C- and N-terminal domains of IL-16 are required for interaction with CD4.
A synthetic peptide representing the 16 C-terminal aa residues of IL-16, Arg106 - Ser121, can inhibit IL-16 chemoattractant activity of CD4+ cells.25 This peptide also competes for binding of the OKT4 anti-CD4 monoclonal antibody suggesting that it is a competitive inhibitor for CD4 receptor binding. The minimal peptide sequence, RRKS (Arg106 - Ser109), can mediate the inhibition of IL-16 chemoattractant activity.26 Specifically, the critical residue in native IL-16 responsible for inducing CD4+ cell migration by binding or activation of the IL-16 receptor appears to be Arg107.26 In support of this observation, Arg107 is conserved in the IL-16 sequence of mouse and various primate species.15
Sources of IL-16 include epithelial cells,13,27-31 mast cells,l13,27,30,32 lymphocytes,17,33-36 macrophages,13,30 synovial fibroblasts,37 and eosinophils.30,38 IL-16 mRNA is constitutively expressed in both CD4+ and CD8+ cells,35 however, synthesis is induced in T lymphocytes upon exposure to antigen or mitogen.1 IL-16 may also be secreted by activated CD8+ cells in response to histamine or serotonin. IL-16 expression has been linked to inflammation processes in asthma,27-31,39 rheumatoid arthritis,37,40 systemic lupus erythematosus,41 colitis,42 atopic dermatitis,43 and multiple sclerosis.44,45 For example, the expression of IL-16 directly correlates with the number of infiltrating CD4+ T cells in asthmatic epithelium.29,46
IL-16 has also been shown to play a role as a suppressive factor for HIV-1. Recombinant IL-16 (C-terminal 130 aa) represses HIV-1 replication in peripheral blood mononuclear cells.47-50 Human CD4+ cells transfected with IL-16 cDNA are resistant to HIV-1 infection, not at the level of viral entry or reverse transcription, but at the level of expression of mRNA.51 Apparently, secretion of IL-16 is required for HIV inhibitory activity since cells expressing either the C-terminal 130 aa or the C-terminal 100 aa without signal peptides are poor secretors of IL-16 and exhibit little, if any, resistance to HIV.52 Although both IL-16 and HIV-1 use CD4 as a receptor, studies have shown that they do not share a common binding site.6,53 The mechanism of HIV suppression by IL-16, therefore, does not involve steric inhibition of viral binding to CD4 as is the case with several CC chemokines (i.e., RANTES, MIP-1 alpha, and MIP-1 beta).54-58 Rather, IL-16 promotes the inhibition of HIV-1 promoter activity.7
The use of IL-16 or IL-16 antagonists may have potential therapeutic value in various pathological immune responses. In HIV-1 infection, IL-16 may be used to promote CD4+ T cell immune reconstitution and repress HIV-1 replication. Analysis of serum samples from HIV-1-infected individuals demonstrate consistent or increased IL-16 levels during the asymptomatic phase and a significant drop upon disease progression.50 Treatment of severely immunodeficient HIV-infected individuals with the HIV protease inhibitor indinavir can reduce plasma HIV RNA levels and result in significant increases in circulating IL-16, RANTES, MIP-1 alpha, and MIP-1 beta levels, which can then act to inhibit further HIV replication.59 Because viral suppression does not depend solely on IL-16 or CC chemokines, the combined therapeutic potential of IL-16 and CC-chemokines may be more effective in suppressing HIV-1.49,60,61 In contrast, IL-16 antagonists may be useful in disease states where the proinflammatory functions of IL-16 are detrimental to the host, such as asthma, rheumatoid arthritis, systemic lupus erythematosus, colitis, atopic dermatitis, and multiple sclerosis. In a mouse model of allergic asthma, anti-IL-16 antibodies significantly inhibited development of airway hyper-responsiveness.28