AKA: T-cell stimulating factor (TSF), natural-killer cell stimulatory factor (NKSF), and cytotoxic lymphocyte maturation factor (CLMF).
Interleukin 12 (IL-12) is an important regulatory cytokine that has a function central to the initiation and regulation of cellular immune responses. It has the capacity to regulate the differentiation of naive T cells into TH1 cells, which is crucial in determining resistance and the type of response that will be elicited in response to a particular pathogen. It stimulates the growth and function of T cells and alters the normal cycle of apoptotic cell death.
Structure and location of IL-12:
IL-12 is one of a large group of cytokines that folds into a bundle of four alpha-helices. It is a heterodimer of 70kDa that is composed of two disulfide-linked subunits, of mass 35kDa and 40kDa. These subunits are coded by different, and seemingly unrelated genes (Brandhuber et al., 1987). Only a single receptor chain has been identified for IL-12, labeled the IL-12Rbeta1 receptor. Itís structure of about 100kDa in humans and mice is most homologous to the leukemia inhibitory factor (LIF) receptor (Chua et al., 1995). IL-12 p40 and p35 chains are encoded by two separate genes that bear no apparent homology. The gene encoding the p40 chain is mapped to chromosome 5q31-q33, a region that encodes many cytokines and cytokine receptors, and the gene encoding the p35 chain is located on chromosome 3p12-3q13.2. The two genes that encode the murine p40 and p35 counterpart chains contain 70 and 60% sequence homology, respectively, to the human genes (Xiaojing et al., 1996)
Detection of IL-12 Gene Expression:
There are three methods to detect the cellular expression of IL-12. They are: Northern blot, RNAse protection, and competitive/quantitative PCR. Polyclonal antibodies to the p40 and p35 chains are also formed to detect the presence of IL-12. The production of IL-12 requires an apparent coordinated expression of both p40 and p35 chains that makes the formation somewhat challenging. Additionally, the p40 homodimer has been shown to have an antagonistic role in the mouse (Gillessen, 1995). Furthermore, the p35 chain of IL-12 has a more widespread expression throughout the individual and its mRNA was found in both brain and lung tissue, while the p40 chain was not detected in either of these areas. It is thought that the p35 chain may have a function within the body independent of its regulatory capacity through IL-12. Upon activation of phagocytic cells, accumulation of IL-12 is observed to be somewhat delayed in relation to the expression of other inflammatory cytokines, sich as TNF-alpha and IL-1beta, and then subsides after several hours. The activation of the p40 chain requires active protein synthesis, mainly at the initiation of the stimulus. The production of IFN-gamma has a very powerful effect in enhancing the ability of phagocytic cells to produce IL-12 and IL-12 also enhances the production of IFN-gamma, creating a positive reinforcement loop. Early induction by IL-12 of IFN-gamma expression is key to the initiation of the innate immune response.
IL-12 has great potential as a vaccine adjuvant for promoting cell-mediated immunity and a TH1 cell response. Not only does immunization with IL-12 as adjuvant promote a long-term and stable TH1 response, it also enhances the primary TH1 response when given in conjunction with other adjuvants. However, O'Garra notes that repeated exposeure to antigen and IL-12 is necessary to establish a stable TH1 response (O'Garra et al., 1996).
Deletion of genes that form IL-12 in gene knockout mice:
Cytokine responses were monitored for IL-12 knockout mice in vivo to further define the important role of the differentiation of naive T cells into TH1 cells. This differentiation helps create a balance between the cell-mediated and humoral immunity. In this experiment, the knockout mice were observed to be completely viable and fertile, and displayed no developmental abnormalities. However, on an immunological level, these mice were noticed to have a reduced capacity to cause a TH1 cell response and also a relative inability to produce IFN-gamma in response to toxins engulfed by phagocytic cells (Magram, et al., 1996).
Function of Co-receptors in the action of IL-12 in mice:
Activated T cells were injected into the mice and the mice were tested for antibodies that might be responsible for the regulation of T-cell responsiveness to the binding of IL-12. CD2 was identified as one of these regulators along with its major ligand, CD58, which binds to its adhesion portion and effectively inhibits the response of the T cells to bound IL-12. However, this regulation does not effect the binding of IL-12 in any way. In fact, the presentation of adhesion molecules by an antigen presenting cell, a monocyte for example, illustrates how these APCís can regulate the response of T cells to a cytokine, without disturbing the cytokine binding interaction (Gollob and Ritz, 1996).
Figure 1: This shows how the binding of the CD-2 ligand, CD-58, is a very important part of the positive reinforcement of the T-cell by the monocyte (Gollob and Ritz, 1996). This interaction causes the production of IFN-gamma by IL-12. Permission is pending from the author of this article for the use of this figure. If not approved for use, it will be removed from the site.
Normal Function/Signal Transduction involving IL-12:
The induction of a cell-mediated immune response to a specific antigen is regulated, for the most part, by the release of cytokines. IL-12 is produced by macrophages, monocytes, dendritic cells, and B cells in response to bacterial products and intracellular parasites. The biological effects of the production of IL-12 are directed at T cells and NK cells. IL-12 is responsible primarily for the subsequent production of IFN-gamma and tumor necrosis factor-alpha (TNF-a) from both NK cells and helper T cells. Researchers have concluded in recent experiments that because IL-12 is responsible for the production of IFN-gamma, itís immunological action must be directed primarily to those cells that are capable of producing IFN-g. The cells that produce IFN-g most often are those activated T cells that also have the coreceptor, CD30, present on their surface. Therefore, CD30+ T cells are a target of the actions of IL-12 (Alzona, 1994). IL-12 also stimulates the rate at which NK cells and helper T cells proliferate following antigen activation. In addition, the lytic capacities of both NK and helper T cells are increased by the presence of IL-12. IL-12 has the specialized function of leading naive CD4+ T cells to differentiate toward the TH1 cell type in order to prepare for the release of IFN-gamma and for the development of the cell-mediated immune response (Hsieh, 1993). However, IL-12 is not effective in the down regulation by means of reversing TH2 cells differentiation. IL-12 and IL-2 are both important cytokines in the regulation of a cell-mediated immune response, IL-2 being responsible for stimulating the growth and proliferation of T cells, while IL-12 stimulates the differentiation of the CD4+ T cells into TH1 cells. The sites of phosphorylation and activation of particular transcription factors during the signaling pathways of these two cytokines are not completely understood and a likely model based on experimental data is shown below.
Figure 2: A model produced from experimental data, indicating the possible signaling pathways for two similar cytokines, IL-12 and IL-2 (Bacon et al., 1996). Permission is pending for this figure from the author of the article. If permission is not granted for use of this figure on this site, it will be removed.
It is also important to note the function that IL-12 has in the regulation of the production of antibody isotypes. The direct binding of IL-12 to B cells causes a long-term enhancement of antibody production, in addition to the isotype switching that is caused by the induction of IFN-g production as IL-12 stimulates the differentiation of TH1 cells (Metzger 114).
Alzona, M., H.M. Jack, R.I. Fisher, and T.M. Ellis. 1994. Journal of Immunology. 153, 2862-2867.
Bacon, C.M., S.S. Cho, and J.J. O'Shea. 1996. Annals of the New York Academy of Sciences. 795, 41-59.
Brandhuber, B.J., T. Boone, W.C. Kenney, and D.B. MacKay. 1987. Science. 23, 1707-1709.
Chua, A.O, V.L. Wilkinson, D.H. Presky, and U. Gubler. 1995. Journal of Immunology. 155, 4286-4294.
Gillessen, S., D. Carvajal, P. Ling, F.J. Podlaski, D.L. Stremlo, P.C. Familletti, U. Gubler, D.H. Presky, A.S. STern, and M.K. Gately. 1995. European Journal of Immunology. 25, 200-206.
Gollob, J.A. and J. Ritz. 1996. Annals of the New York Academy of Sciences. 795, 71-81.
Hsieh, C.S., S.E. Macatonia, C.S. Tripp, S.F. Wolf, A. O'Garra, and K.M. Murphy. 1993. Science. 260, 547-549.
Magram, J., J. Sfarra, S. Connaughton, D. Faherty, R. Warrier, D. Carvajal,
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the New York Academy of Sciences. 795, 60-70.
Metzger, D, J.M. Buchanan, J.T. Collins, T.L. Lester, K.S. Murray, V.H. Van Cleave, L.A. Vogel, and W.A. Dunnick. 1996. Annals of the New York Academy of Sciences. 795, 100-115.
O'Garra, A., B. E. Murphy, K. Shibuya, N. Hosken, P. Openshaw, V. Maino, K. Davis, and K. Murphy. 1996. Reversibility of T helper 1 and 2 populations is lost after long-term stimulation. Journal of Exp. Medicine. 183, 901-913.
Xiaojing, M.A., Miguel Aste-Amezaga, and Giorgio Trinchieri. 1996. Annals of the New York Academy of Sciences. 795, 13-25.
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