MCPIP1 (<i>Zc3h12a</i>) keeps inflammation in check by cleaving 3′ UTRs (2013)

Inflammation, like many processes in the immune system, is a double-edged sword. On the one hand, the inflammatory state, mediated by activated resident or infiltrating immune cells that produce effector molecules such as nitric oxide, prostaglandins, cytokines and chemokines, is crucial to the clearance of pathogens. Yet, too much inflammation can also cause damage to host tissues, resulting in release of alarmins or damage-associated molecular patterns, and in extreme cases death. A growing list of inflammatory and autoimmune disorders demonstrates the difficulty of controlling inflammation once regulatory mechanisms have been subverted. In this issue, Miao et al. provide evidence that negative regulation of the immune response through MCPIP1 is critical for tolerance to commensal bacteria.1 Its mechanism of action, as outlined below, is novel and should provide a useful paradigm for understanding the complex regulation of inflammation and immune system homeostasis. The dramatic inflammation observed in MCPIP1-deficient animals suggests that the immune system is constantly being activated by commensal bacteria, and MCPIP1 is required to prevent it from overreacting to otherwise benign microbiota. In support of this notion, survival of MCPIP1-deficient mice can be improved by antibiotic treatment.1 MCPIP1 is encoded by a gene, officially named Zc3h12a, that is induced by monocyte chemotactic protein-1 (MCP-1, aka CCL2) in human peripheral blood monocytes.2 MCPIP1 is upregulated within hours in macrophages in response to Toll-like receptor (TLR) and interleukin-1 (IL1) signaling via MyD88, and functions as a negative regulator of the inflammatory response through multiple mechanisms.3, 4 MCPIP1 was initially predicted to affect mRNA stability due to its CCCH-zinc-finger domain that is reminiscent of other RNA-binding proteins (for example, TTP and Roquin), which also exhibit negative inflammatory effects. Indeed, MCPIP1 binds to a conserved stem-loop element (∼30 nt) within the 3′ untranslated region (UTR) of Il6, and another similar element in Zc3h12a itself.4, 5 However, the molecular basis of MCPIP1 interaction with RNA remains unclear, since no sequence specificity has been found and structural studies have not been performed. Yet, it has been shown to degrade cytokine mRNAs by acting as an endonuclease,4 in contrast to TTP and Roquin that need to recruit other proteins that catalyze mRNA decay. Furthermore, MCPIP1 does not appear to depend on adenine- and -uridine-rich elements.4 MCPIP1 can also inhibit miRNA biogenesis by recognition and subsequent cleavage of the stem-loop structure of select precursor miRNAs,6 and recently, it was shown to bind and degrade viral RNA.7 Furthermore, there is evidence that MCPIP1 can mediate inhibition of JNK and nuclear factor-κB signaling by de-ubiquitylating the tumor necrosis factor receptor-associated factor (TRAF) family of signal-transduction proteins, although the molecular basis for its proposed de-ubiquitylation activity has not been fully defined.8 In this report, Miao et al.1 provide further characterization of disease progression in MCPIP1-deficient animals, showing increased levels of cytokines, produced by both innate and adaptive immune cells, and eventually fatal inflammation. These increased cytokine levels, as well as hyperimmunogobulinemia, can be accounted for by the increased percentages of activated T cells, plasma cells and neutrophils observed.1, 4 It would be interesting to find out if follicular T helper cells are expanded in this case. In spite of the aggressive inflammation seen, the Foxp3+ regulatory T-cell population is maintained, although their suppressive capacity has not been determined. Most notably, the MCPIP1 knockout animals studied exhibit aggressive infiltration of activated immune cells into non-lymphoid tissues as well as progressive disruption of secondary lymphoid organ architecture. This group has previously reported that MCPIP1 expression is highest in lymphoid organs and mucosal surfaces.8 This mucosal localization could suggest a role for negative regulation by MCPIP1 in the first phases of the inflammatory response at the site of pattern-recognition by innate receptors. The new findings of Miao et al.1 are consistent with this, and their study suggests that commensal bacteria may drive the uncontrolled inflammation in MCPIP1-deficient mice. Future studies could employ germ-free mice that can then be recolonized with specific bacteria, or challenged with TLR ligands, in order to better understand the pathogenesis that ensues in the absence of MCPIP1 on a cellular and molecular level. The severe phenotype observed in MCPIP1-deficient mice further demonstrates its non-redundant function in inflammatory regulation. Indeed, without MCPIP1, mice are hypersensitive to septic shock.9 One may postulate that loss of negative regulation, like that of MCPIP1, is actually necessary for septic shock to occur. This lack of redundancy is in spite of three closely related paralogs: Zc3h12b, Zc3h12c and Zc3h12d. These proteins are highly conserved but lack the Proline-rich domain required for MCPIP1 (Zc3h12a) oligomerization.7 Although these paralogs all possess the Mg-dependent PilT N-terminus (PIN) domain and aspartic acids for catalysis, they have not yet been confirmed to act as RNases.3, 4 Interestingly, although Zc3h12b and Zc3h12c are upregulated in Zc3h12a-deficient spleens, they fail to rescue the hyperinflammation.1 However, as previously discussed, the spleen may not be the relevant organ to trigger the pathogenesis. Further studies on these paralogs may yield yet unknown mechanisms of immunomodulatory regulation. Despite increased understanding of MCPIP1 function, these studies raise more questions about its precise role in the inflammatory process. Studies have mainly focused on innate cells and mice completely devoid of MCPIP1. It would be important to know which specific lineages are affected in a cell autonomous fashion so that we can begin to understand which effects are primary, as opposed to secondary. For example, in a mouse septic shock model, it seems that MCPIP1 function in macrophages is crucial, but it would be informative to investigate other cell types.9 Inflammation dependent on the microbiome has been shown in several model systems.1, 10 Yet, the precise microbiome-derived signals perceived as ‘dangerous’ that trigger inflammation in the absence of MCPIP1 remain unclear. It is possible that in the absence of MCPIP1, the microbiome has evolved and is no longer benign—a phenomenon referred to as dysbiosis. Recent studies also bring into question the possibility of a sex bias in these models.11 These interesting series of studies on MCPIP1 led us to propose the following question and working model. Could the induction of negative feedback loops, such as the one mediated by MCPIP1, enforce a cyclic nature to the inflammatory response? An inflammation limiting re-inducible cycle could allow pathogen clearance, and initiation of tissue repair, while maintaining inflammation below the point of host tissue damage (Figure 1). Finally, these mouse studies make the valuable prediction that loss-of-function mutations in MCPIP1 in humans could result in chronic inflammation or autoimmunity. MCPIP1 mediates a negative feedback loop essential for immune tolerance to commensal bacteria. An abbreviated regulatory circuit is depicted, whereby MCPIP1 post-transcriptionally represses inflammation upon NF-κB activation by cleaving the 3' UTRs of itself (Zc3h12a), cytokine transcripts and miRNA precursors. The model depicts the inflammatory cycle imposed by the self-limiting negative feedback loop enacted by MCPIP1. Modulation of the inflammatory response by MCPIP1 protects host tissues while allowing for pathogen clearance. Failure to clear pathogen would reactivate the cycle, while still protecting host tissues (blue arrow). Loss of MCPIP1 results in an unregulated inflammatory response (red dashed line), and crosses a threshold leading to chronic inflammation and perhaps autoimmunity.