Trends in Immunology
ReviewIL-12p40: an inherently agonistic cytokine
Section snippets
IL-12p40, the hidden protagonist
The importance of the interleukin (IL)-12 family of cytokines in molding the effector functions of lymphocytes is well established 1, 2, 3. However, the subunit IL-12p40, which contributes to both the IL-12 and IL-23 heterodimers, is not often considered as an intrinsically functional cytokine. When it is considered, it is as an antagonist because the recombinant murine (rm)IL-12p40 homodimer (IL-12p80) competitively binds to the common receptor component IL-12Rβ1 4, 5, 6 and prevents
The IL-12 and IL-12 receptor family
IL-12p40 is produced primarily by activated inflammatory cells including macrophages, neutrophils, microglia and dendritic cells (DCs) when these cells are stimulated by a variety of pathogenic or inflammatory agents 1, 2. It can also be produced by keratinocytes [10] and airway epithelial cells [11]. IL-12p40 is a component of IL-12p70 and IL-23 (Figure 1), and both IL-12p35 [12] and IL-23p19 [13] require IL-12p40 to be secreted. However, IL-12p40 can be secreted as a monomer (IL-12p40) and as
Signaling through the IL-12 receptor family
Although IL-12Rβ2 and IL-23R have long cytoplasmic tails and mediate intracellular signaling, the IL-12Rβ1 cytoplasmic tail is short, does not contain tyrosine residues [21] (Figure 1) and has a limited signaling role [23]. In lymphocytes, the IL-12R and IL-23R complexes activate the Janus kinases Tyk2 and Jak2 and the transcription factor STAT4 [24]. Although IL-12Rβ2 and IL-23R activate Jak2 upon ligation [24], the cytoplasmic portion of IL-12Rβ1 alone can phosphorylate Tyk2 and STAT3 [25].
IL-12p40/p80 can act independently to promote cellular responses
The role of IL-12p80 as an antagonist of IL-12p70 function was first proposed by Gately and colleagues [5] and supported by the fact that exogenous rmIL-12p80 protects mice from IL-12-dependent but not TNF-dependent shock [7]. This cytokine also blocks in vivo interferon (IFN)γ production [16] and inhibits IFNγ synthesis from splenocytes treated with IL-12p70 [4]. Furthermore, in transgenic IL-12p40 mice, IFNγ is suppressed in a model of hepatic infection [34] and lupus-like autoimmune disease
What is required to confirm an independent role for IL-12p40/p80?
To determine how IL-12p40/p80 acts, we need to determine its effects on individual cells. However, there are some difficulties. Much of the early data supporting an independent role for IL-12p40 was published before the discovery of IL-23 and therefore independent roles for IL-12p40 and IL-23 were not distinguished. The differences seen between IL-12p35- and IL-12p40-deficient mice are a prime example of how IL-23 and IL-12p40/80 could both mediate non-IL-12p70 activity 42, 44. The mouse
IL-12p80 as a macrophage chemoattractant
One of the earliest established activities of IL-12p80 is as a chemoattractant for macrophages [47]. Recombinant murine IL-12p80 acts as a chemoattractant for both rat and mouse macrophages over a dose range of 10–1000 ng/ml; this activity is blocked by the anti-IL-12Rβ1 antibody and is lost following boiling [47]. Chemotactic activity is reduced when five amino acids are removed from the carboxy-terminal end of IL-12p40 [47]. In the same study, the induced expression of rIL-12p40 by a hepatoma
IL-12p80 as an inducer of DC migration
Recent work on the induction of pulmonary acquired cellular responses has identified a novel role for IL-12p40, probably in the IL-12p80 form, as an inducer of DC migration [37]. In this work the migration of DCs from the lung to the draining lymph node in response to a mycobacterial challenge is defective in mice lacking IL-12p40 but not in those lacking IL-12p35. Specifically, IL-12p40-deficient DCs are unable to migrate from the lungs to the lymph node in response to mycobacteria. The
IL-12p40/p80 can mediate inflammatory responses in the lung
An association between IL-12p40/p80 and potentially damaging inflammatory responses within the lung has been reported. Specifically, IL-12p40 and IL-12p80, as identified using western analysis, are seen at high levels in the BAL of asthma patients and are associated with macrophage accumulation [17]. Using the mouse model, it was shown that the airway epithelium expresses IL-12p40 following exposure to TNF or Sendai virus and that both IL-12p40 and IL-12p80, but little p70, are present in the
IL-12p40 in allograft rejection
IL-12p80 has been implicated in allograft rejection, in two models, because of its ability to induce IFNγ production by CD8+ T cells [58] and because macrophages accumulate in the grafted tissue [11]. In the first model, IL-12p80 acts as an agonist on CD8+ T cells, which is in contrast to its other agonist activities when it acts on macrophages and DCs. Thus, when delivered in vivo, rmIL-12p80 promotes allograft rejection [36] but, when CD8+ T cells are depleted, rmIL-12p80 promotes allograft
Concluding remarks and future perspectives
In conclusion, the ability of rmIL-12p40/p80 to promote DC migration, macrophage inflammation and the production of IFNγ in CD8+ T cells identifies this cytokine as an immunostimulatory agent (Figure 2). It is particularly timely and important that we understand this agonist activity of IL-12p40/p80 in the light of using anti-IL-12p40 antibodies to treat inflammatory diseases [60]. IL-12p80 is the primary mediator in mice and its presence in human BAL supports further searches for IL-12p80 in
Acknowledgements
We have been supported by NIH grants AI46530 and AI067723. S.A.K. was supported by a New York Community Trust–Heiser Fund fellowship. We thank John Pearl for generating Figure 1.
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