Trans–mission control in the urinary tract: Local cytokine regulation of monocyte proliferation to combat infection

Summary Sentence:

Discussion on in situ monocyte proliferation in response to IL-6 trans-signaling during a urinary tract infection.

Urinary tract infections (UTIs) are one of the most common bacterial infections worldwide, affecting approximately half of all women during their lifetime. The majority of infections are acute and self-limiting due to the rapid and potent host immune response. At the frontline of these host defenses is the production of the pro-inflammatory cytokine interleukin-6 (IL-6), which is important for recruiting immune cells from the periphery to the bladder mucosa. IL-6 has long been identified as a highly induced cytokine during UTIs in both mice and humans [1]. Bladder epithelial cells are known to produce large amounts of IL-6 in response to infection with uropathogenic E. coli (UPEC), the most common culprit causing UTIs. Furthermore, some UPEC strains dampen IL-6 secretion from these cells, a virulence strategy hinting at IL-6’s importance in eliminating the bugs from the bladder mucosa [2]. Nevertheless, the role IL-6 plays in in vivo host defense, particularly in the context of UTIs, continues to be elucidated. In this issue, Dixit et al. [3] shed new light on IL-6 activity during UTIs in mice. They examined cytokine control of in situ monocyte proliferation and elucidated how the initial defenses are orchestrated. Their results demonstrate that local production of IL-6 induces proliferation of monocytes that have infiltrated the bladder mucosa. This new work adds to the growing list of functions served by the pleiotropic cytokine IL-6, as well as to our understanding of the immune response to bladder infection.

Despite the well-known and seemingly commonplace role of IL-6 in host defenses, control of IL-6 production is not well understood due to its multiple functions and signaling modalities [4]. Classical IL-6 signaling through membrane-expressed IL-6 receptor (IL-6R) and its signal transducer gp130 is restricted to hepatocytes, which coordinate the acute phase response, and to hematopoietic cells. However, nearly all other cells in the body express gp130 in the absence of IL-6R and respond to IL-6 via trans-signaling, which requires IL-6 to be bound to soluble IL-6R. Furthermore, IL-6 has both pro-inflammatory functions (e.g. stimulating granulopoiesis, neutrophil respiratory burst, and hyperthermia) as well as anti-inflammatory functions (e.g. mediating neutrophil apoptosis, inhibiting further chemokine and cytokine expression, and resolving inflammation). These seemingly antagonistic properties have made dissecting the role of IL-6 during infections challenging; IL-6 may have drastically different functions throughout the course of infection, impact different cell types, and act via different signaling mechanisms.

Here, Dixit et al. show that locally produced IL-6 induces leukocyte antigen 6 complex, locus 1+ (commonly referred to as Ly6C+) monocyte proliferation within the inflamed bladder mucosa. In a seminal paper, the same group previously demonstrated that infiltrating Ly6C+ monocytes are a key component in coordinating an effective immune response against UPEC [5]. In that work, Ly6C+ monocytes were shown to produce TNFα during UTIs. Interestingly, monocyte-derived TNFα indirectly licensed neutrophil transepithelial migration by first acting on resident Ly6C- macrophages, stimulating them to secrete CXCL2. Neutrophils then responded to CXCL2 by secreting MMP-9, which degrades the uroepithelial basement membrane, allowing neutrophils access to infected superficial urothelial cells and the bladder lumen. In the current work, the group identified in situ proliferation of infiltrating, inflammatory Ly6C+ monocytes. While these monocytes are known to proliferate in the bone marrow and migrate to tissues during inflammation in a CCL2/CCR2-dependent manner, their proliferation within inflamed tissues in response to infection has not been well-described. In contrast to the infiltrating cells, Dixit et al. showed that Ly6C-F4/80+ resident macrophages demonstrated only minimal proliferation during acute infection. Importantly, they were able to replicate their results in a LPS-induced peritonitis model, indicating that in situ monocyte proliferation is not limited to the bladder and may be a feature of the innate immune response to local infection throughout the body. Furthermore, they showed that Ly6C+ monocyte proliferation was dependent on IL-6 trans-signaling rather than classical IL-6 signaling and that local IL-6 production was primarily produced by non-hematopoietic cells (likely bladder epithelial cells). However, while the work provides strong evidence for in situ monocyte proliferation during UTIs, whether blocking such proliferation affects the outcome of UTIs need to be further explored. Similarly, how local monocyte proliferation affects the duration and severity of sterile inflammation (modeled here by LPS-induced peritonitis) should be further investigated.

The role of monocytes in UTIs is somewhat controversial. Five different groups studying UTIs in mice have depleted monocytes using clodronate liposomes, which deplete phagocytes such as blood monocytes and, to a lesser extent, resident tissue macrophages. Interestingly, the results are not entirely consistent among studies, indicating an incomplete understanding of the role these cells play during UTIs. In one study, depleting monocytes with clodronate liposomes decreased the bacterial burden in the bladder at 24 hours post-infection, but CCR2-/- mice, which are unable to recruit monocytes to the inflamed tissue did not show a decreased bacterial burden [6]. These results suggest that monocytes might exacerbate UTIs rather than contribute to its resolution. In support of that hypothesis, another group observed a reduction in inflammation at 24 hours post-infection as well as a reduction in chronic cystitis at 4 weeks post-infection in clondronate-treated mice, but did not identify changes in bacterial burden in the bladder at 24 hours post-infection [7]. In contrast, the seminal paper described above demonstrated a critical role for recruited monocytes in coordinating the action of neutrophils and resident macrophages during UTIs [5]. When that group used clodronate liposome depletion or CCR2-/- mice, they found an increased bacterial burden in the bladder 24 hours post-infection. In support of those findings, another group showed treatment with clondronate liposomes resulted in an increased bacterial burden in the bladder, but no difference in the bacteriuria titers at 24 hours post-infection [8]. In yet another study, in mice deficient in ATG16L1 (an essential autophagy protein), which exhibit an enhanced ability to clear UTIs, clodronate liposome treatment reversed this enhanced clearance, suggesting a role for monocytes in UPEC clearance [9]. While these discrepancies could be due to differences in UPEC strains, mouse strains, or experimental procedures among labs, the varying results highlight the need for further investigation into the role of the monocyte/macrophage lineage during an acute UTI and in the post-infection tissue reparative process in the bladder mucosa.

Interestingly, the current study used instillation of anti-IL-6, anti-gp130, and soluble gp130 (sgp130) directly into the bladder to investigate the role of IL-6 in the local tissue. The success of this technique indicates that locally applied antibody therapies may be effective in treating inflammatory disorders of the bladder, such as chronic/recurrent UTIs and interstitial cystitis/bladder pain syndrome. These disorders are particularly prevalent among post-menopausal women, who have decreased estrogen, a well-known repressor of IL-6 expression. We have previously shown that estrogen deficiency secondary to ovariectomy delays bacterial clearance from the bladder and results in increased inflammation, IL-6 levels, and quiescent intracellular reservoirs that can seed chronic and recurrent UTIs [10].

Anti-IL-6 therapy via bladder instillation could potentially alleviate the inflammatory component of these diseases while reducing systemic side effects. Tocilizumab, a monoclonal antibody against IL-6R, is already used clinically to treat rheumatoid arthritis (RA) and other chronic inflammatory diseases [4]. Whether inhibiting monocyte proliferation in inflamed synovial joints contributes to the efficacy of tocilizumab in RA is not currently known. This work suggests that targeting specific IL-6 signaling mechanisms, in this case IL-6 trans-signaling via sgp130, could be an effective strategy to control monocyte-driven inflammation without disrupting essential functions of classical IL-6 signaling. Clearly, more work would need to be done to establish the safety and efficacies of these potential new therapies. While many questions remain regarding the complete and dynamic role of IL-6 during bacterial infections, this new work sheds light on at least one new function of IL-6.

Figure 1. Interleukin-6 response to acute urinary tract infection with uropathogenic E. coli (UPEC).

Model shows urothelial cells secrete IL-6 and Ly6C- resident macrophages secrete CCL2 and other chemokines. These early signals recruit inflammatory LyC6+ inflammatory monocytes and neutrophils to the bladder mucosae. Ly6C+ monocytes respond to local IL-6 via trans-signaling to proliferate in situ.

Abbreviations

ATG16L1

autophagy related protein 16-like 1

CCL2

chemokine (C-C motif) ligand 2

CCR2

C-C chemokine type receptor 2

CXCL2

chemokine (C-X-C motif) ligand 2

F4/80

epidermal growth factor-like module-containing mucin-like hormone receptor-like 1 (EMR1/ADGRE1)

gp130

glycoprotein 130

IL-6

interluekin-6

IL-6R

IL-6 receptor

LPS

lipopolysaccharide

Ly6C

leukocyte antigen 6 complex, locus 1

MMP-9

matrix metalloproteinase 9

RA

rheumatoid arthritis

sgp130

soluble gp130

TNFα

tumor necrosis factor alpha

UPEC

uropathogenic E. coli

UTI

urinary tract infection