Structure

IL-16 was originally identified as a homotetramer consisting of individual 14 kDa monomers of 130 amino acids (aa) each. IL-16 is synthesized as a precursor molecule (pro-IL-16) of approximately 68 kDa and 631 aa lacking a signal peptide. The gene for IL-16 maps to chromosome 15 in the human and chromosome 7 in the mouse. 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. Human and mouse IL-16 homologs display conservation of both structure and function, particularly in the C-terminal region (82.1% similarity). The human pro-IL-16 sequence displays >90% similarity to various non-human primates.

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. 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). The mechanism of release or secretion of IL-16, however, is currently unknown but does not appear to correlate with apoptosis.

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. 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. 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.

How it is signaling

CD4 serves as a signal-transducing receptor for IL-16. Expression of CD4 is required for mediating IL-16 functions. Interaction between IL-16 and CD4 can specifically initiate an increase in intracytoplasmic calcium and inositol trisphosphate, activation of p56lck, and translocation of protein kinase C from the cytosol to the cell membrane. IL-16-induced functions can be inhibited by a monomeric Fab of an anti-CD4 (OKT4) monoclonal antibody. The region of CD4 that binds IL-16 has been identified within the D4 domain, overlapping structure involved in CD4 dimer formation. 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. 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. 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. In support of this observation, Arg107 is conserved in the IL-16 sequence of mouse and various primate species.

Bioactivity (posted on R&D)

Sources of IL-16 include epithelial cells, mast cells, lymphocytes, macrophages, synovial fibroblasts, and eosinophils. IL-16 mRNA is constitutively expressed in both CD4⁺ and CD8⁺ cells, however, synthesis is induced in T lymphocytes upon exposure to antigen or mitogen. 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, rheumatoid arthritis, systemic lupus erythematosus, colitis, atopic dermatitis, and multiple sclerosis. For example, the expression of IL-16 directly correlates with the number of infiltrating CD4⁺ T cells in asthmatic epithelium. 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. 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. 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. Although both IL-16 and HIV-1 use CD4 as a receptor, studies have shown that they do not share a common binding site. 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). Rather, IL-16 promotes the inhibition of HIV-1 promoter activity.

To conclude:

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. 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. 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. 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.