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FASEB J. Dec 2011; 25(12): 4073–4078.
PMCID: PMC3470729

Inflammatory Gout: Observations over a Half-Century


This is a discussion of acute gouty arthritis, seen for over 50 years of engagement. It addresses the evolution of our current understanding of the interaction between urate crystals and key cellular components of the gouty inflammatory paroxysm, with new material on pathogenesis.

figure z38012118607r001
Bunbury's Dream: “Funny, the faster he makes them go, the worse my gout gets.” Color image from Bunbury, Henry William, 1750–1810: Origin of the Gout.

Although acute gouty arthritis has been known as a clinical entity for millennia, the most perceptive, early insights into its pathogenesis came from A. B. Garrod, who in the mid-19th century, convincingly placed crystals of sodium urate at the center of gouty inflammation (1). However, as late as 1953, the definitive treatise on synovial fluid changes in joint diseases, in a close analysis of the contents of joint fluid in gout, made no mention of the presence of urate crystals (2). The modern study of acute gouty arthritis began with McCarty and Hollander in 1961, who used polarized light microscopy to demonstrate that the filamentous strands in gouty synovial fluid, unseen or ignored in light microscopy, are crystals of monosodium urate (MSU) (3), often within neutrophils (4, 5). Since then, the presence of urate crystals in joint fluid has made the diagnosis of gout.

We began working on gout at approximately that time, and the evolution of our thinking about it follows. We will deal here primarily with human subjects and materials. Basically, our focus moved from inflammatory properties of urate crystals in man, to the critical role of neutrophils in the inflammatory process, to the governing role of mononuclear phagocytes in the acute attack. We conclude with a somewhat altered view of the mechanics of acute gouty arthritis, based on preliminary but dramatic videomicroscopic findings. The new data suggest that a subset of mononuclear phagocytes triggers gouty arthritis by interacting with crystals and attracting neutrophils. The attracted neutrophils have not yet ingested crystals. On arriving, the neutrophils cleave to the body of the monocyte, not to protruding crystals. We believe that ingestion of crystals by neutrophils—a hallmark of acute gout in joint fluids—may be one of their less important very early functions in the propagation of the inflammatory response.


What brings the subject up now is riveting work on the identification of urate crystals as cellular danger signals (6). Based primarily on work in mouse monocytes, ingested crystals were thought to launch acute gout through the caspase 1-activated Nlrp3 (formerly Nalp3) inflammasome, an intracellular platform whose assembly leads to the conversion of pre-IL-1β to IL-1β and secretion of the latter (7). A number of clinical facts have been used to bolster this vision but do not. A recent example is whether igniting the inflammasome in gout requires two signals or one. Those who support two signals (urate crystals plus something else—e.g., LPS, TNF-α, fatty acids) point to the frequent lack of inflammation associated with urate crystals packed into deposits (tophi), as opposed to free in tissues, and the failure of injected urate crystals in some models to induce inflammation (8). The collateral assumption is that anyone's crystals that seem to induce inflammation alone are probably contaminated with endotoxin—a second signal (9). Let's look at that.


At the beginning of the 1960s in the Seegmiller laboratory at NIH, we injected sterile, pyrogen-free urate crystals, of the configuration found in gout, into one knee of human volunteers, and an amorphous-appearing (actually micromicro urate crystal) preparation into the other knee (4). The former induced inflammation; the latter did not. The same was true with s.c. injections (10). As in gout, inflammation was diminished by pretreatment with colchicine (10). The crystals showed an X-ray diffraction pattern identical to that of urate deposited in gouty tophi (11)—thus, same subjects, same urate chemically, different urate physically, different result. At the same time, McCarty induced inflammation in his own and a clinical fellow's knees with urate crystals (12). We will talk about tophi later, but whatever the role of the Nlrp3 inflammasome may be, stimulation by crystals alone—if they are the right crystals—would seem to be sufficient to induce inflammation in human joints. What triggers this response in vivo is another matter (13).


In the early observations of gouty synovial fluid ex vivo, the neutrophil counts and numbers of neutrophil-associated crystals generally correlated with the degree of inflammation (4, 5). Until the importance of monocytes in gouty inflammation began to be appreciated a generation later (14), neutrophils in their interaction with urate crystals were the cells at the center of gouty inflammation—crucial, then as now, for their proinflammatory functions.


We celebrate phagocytosis of crystals by neutrophils because that is what we see after the fact. But neutrophils do not readily ingest urate crystals in vitro (15), and it is fair to ask whether they function in vivo primarily as extracellular agonists. Favoring this view is the rapidity of the biochemical responses elicited by crystals in human neutrophils. Most of them are evident within <30 s, when there is little, if any, evidence of internalization of crystals (16). We have reviewed in detail elsewhere current knowledge of the nature of the interactions between neutrophils and urate crystals. These include membrane perturbations, the resulting proinflammatory neutrophil responses (degranulation of primary and secondary granules, activation of the NADPH oxidase, the synthesis and release of chemokines, and other factors), and the biochemical signaling pathways involved (17).


Paul Klemperer, the great Vienna-born pathologist at Mt. Sinai Hospital in New York, used to warn his students: “Don't tell me what you know, tell me what you see. Then we'll break our heads.” Here, we examine how leukocytes actually behave in the presence of crystals, as viewed directly in the videomicroscope. We will begin with neutrophils.

Although urate crystals activate the chemotactic factor C5a from serum (18, 19), we see here another example where chemoattraction and even close association between target and cell are not tantamount to ingestion (Fig. 1). Neutrophils tend to “flirt” with crystals; they approach, contact, even envelop them, but then may retreat and return. They do not approach crystals in decomplemented serum (not shown). Contrast this with zymosan particles, commonly used in studies of phagocytosis, which also activate complement. Presented to neutrophils in this system, they are rapidly ingested (not shown). Why then do neutrophils ingest as many crystals as we often see in gouty synovial fluid? We supposed that in vivo, the neutrophils have phagocytic help (below).

Figure 1.
Neutrophils in serum buffer are attracted to urate crystals but often do not ingest them (flirtation). For over 9 min, a neutrophil approached and contacted the crystal shown and then left it but returned (A) and enveloped it over 1-min time (B and C ...


Twenty years after the neutrophil hypothesis for the initiation and propagation of acute gouty inflammation, we tested the generation of “endogenous pyrogen” (EP; later renamed IL-1β) by leukocytes given urate crystals. We incubated leukocytes with urate crystals overnight and injected the supernatant into an ear vein of a rabbit. The readout was the height of a typical fever curve. In this system, virtually all of the EP was coming from the few percent of monocytes among the leukocytes rather than from the neutrophils (14). [Again, as this was in the laboratory of Elisha Atkins, devoted to the study of endogenous pyrogen, measures against contamination with exogenous pyrogens (e.g., LPS) were extreme (14).] Later, using a T cell comitogenic assay based on a murine Th cell clone that does not respond to IFN-α or to TNF-α, we found the release of IL-1 activity from human blood monocytes, as well as from synovial fluid mononuclear cells following stimulation with urate crystals (20). Crystal-induced supernatants with IL-1 activity were neutralized with rabbit antiserum to human IL-1. These supernatants also stimulated the growth ([3H]thymidine incorporation) of long-term, fibroblast-like cell lines derived from human synovial rheumatoid exudates.


Thus, although neutrophils are surely creating the bulk of the inflammatory mischief through the release of their proinflammatory components, we now ascribed initiation of the gouty paroxysm largely to the interaction of mononuclear phagocytes with crystals. Follow-up studies suggested why tophi are generally not proinflammatory. Aggregated urate crystals, as one might find in tophaceous material, were poorly pyrogenic. On ultrasonication, the aggregated urate crystals became first more pyrogenic and then less so as the crystals were dispersed and broken down (13); i.e., they became capable of generating cytokine production when their configuration resembled that of crystals seen in acute gout. It had long seemed unlikely that crystals per se were “both spark and fuel for the inflammatory engine” (21). Based on the 1985 work, we could then suggest how various vicissitudes (serum urate up or down, trauma, fever elsewhere) might trigger gouty attacks and how acute gout might spread to other joints (13).

In the third generation after McCarty and Hollander, there is increasing and convincing evidence that mononuclear phagocytes are driving gouty inflammation—i.e., driving the neutrophil response—throughout its course [reviewed by Martin and Harper (22)] and perhaps even eventually ending it (23). Various mononuclear phagocytes (monocytes, macrophages, type A synovial lining cells) have long been known to be present in synovium in acute gout, even early in the attack, along with the intense neutrophil infiltration (24).


Thus, the finding in synovial fluid of crystals, free or inside neutrophils—crucial diagnostically—may have been somewhat misleading pathogenetically. Synovial fluid, like urine, can reveal only so much about the tissue from which it is derived. The seat of inflammation is perforce the synovium, where the blood vessels are. Neutrophils will dominate the synovial exudate because the inflamed synovium contains so many of them (24) but also because they are major exudative cells. And we suspect further that the ingestion of crystals by neutrophils, which we do see on aspiration of joints, is more likely achieved earlier in the confines of the extracellular synovial space than floating in joint fluid.


What is the nature of the help that we supposed neutrophils to have for uptake of crystals in vivo? In establishing a phagocytic system to address this question in vitro, we looked at a number of possible variables of uptake (Materials and Methods). In Fig. 2, the medium contained 10% autologous serum/buffer, except as noted. Fig. 2A, serum: neutrophils take up crystals better in 10% serum/buffer than in buffer alone [many biochemical and signaling studies are done with no added protein in the medium (25)]. Fig. 2B, complement: plasma-opsonized urate crystals are likely to have adsorbed complement components, including C1q, C1r, and C1s (26), so that complement receptors are likely to be involved in crystal uptake. Neutrophils and monocytes take up crystals better in 10% fresh serum/buffer than in decomplemented (56°C, 30 min) serum/buffer. Monocytes take up crystals much more readily than do neutrophils. Fig. 2C, help for neutrophils by monocyte-conditioned media: incubation of purified monocytes with crystals in serum for 3 h was sufficient to generate conditioned media that, subsequently added to autologous neutrophils, results in the latter taking up more urate crystals than controls; Fig. 2, D–E, help for neutrophils by defined monocyte products: preincubation of cells with the monocyte products IL-1β (100 ng/ml, 30 min) or GM-CSF (300 pM, 45 min) also resulted in the neutrophils taking up more urate crystals than controls. These products did not stimulate uptake of crystals by monocytes significantly (not shown).

Figure 2.
Some contributors to crystal uptake by neutrophils in vitro. Uptake is more efficient with (A) serum, (B) activable complement, (C) monocyte/crystal-conditioned media, and (D and E) the monocyte products IL-1β and GM-CSF.


If neutrophils only flirt with crystals in vitro, then they are devoted to certain mononuclear phagocytes that are associated with urate crystals. Figure 3 shows such a monocyte. The chemoattraction for neutrophils may go on for >2 h (the longest followed). The neutrophils, without intracellular crystals of their own at this point, are not competing for the protruding, monocyte-associated crystals; rather, they are directed at the body of the monocyte. In Fig. 4, the cell with associated crystals is dying. It swells, losing large portions of its membrane like blown bubbles; they are ignored by neutrophils. The monocyte's core (or corpse) continues to attract neutrophils; n.b., we have used “neutrophils” throughout to represent PMN leukocytes, but the attracted cells include eosinophils (Fig. 3) and—more often than one would expect from their numbers—basophils (not shown).

Figure 3.
Monocyte with crystals attracts neutrophils (commitment). (A) Monocyte (m) is ingesting crystals. (B) +12 min: neutrophils have been arriving, including an eosinophil (e). Some have crystals attached to their tails but not ingested. They swarm about in ...
Figure 4.
Monocyte with crystals continues to attract neutrophils postmortem. (A) Two monocytes; one with crystals. (B) + 2 min: cell membrane of the latter swells and detaches as large vesicles (the cell is dying), but (C) 22 min later, the cell body continues ...


What might these hyperactivated mononuclear phagocytes tell us about how acute gout works? They are rare—not seen in the very large majority of preparations—but they are dramatic and suggestive enough, we believe, to deserve the reader's attention. The operant cells in gout need to be defined further, and what follows is speculation. First, our hyperactivated mononuclear phagocytes are monocytes that come from donor blood, not from synovium, where resident cells will have their own characteristics. So far, we see this effect more often when the crystal-associated monocyte begins to die, as in Fig. 4. This could suggest that intact hyperactive monocytes derive their power from the expulsion of intracellular material, for example, via exosomes or more generally, membrane vesicles (27). We are currently examining the effect of various cytokine programs on the production of these cells. These are the kind of external signals that are likely to trigger acute gout (13).


If our cells are indeed indicative of what happens in acute gouty arthritis, then they point to the following sequence of events. Triggered in a number of ways (13), certain resident mononuclear phagocytes begin to take up urate crystals. These cells activate and attract neutrophils. We cannot rule out the neutrophils having also been activated by contact with crystals, but they need not have ingested them at this point. Thus, driven, the neutrophils propagate the inflammatory paroxysm, which is exacerbated further by their own ingestion of crystals, also encouraged by the hyperactive mononuclear cells. When mononuclear phagocytes stop driving the neutrophils, the inflammatory response withers, and acute gouty arthritis subsides.

In summary, work a half-century ago established that urate crystals of the configuration found in acute gouty arthritis can provoke inflammation in human joints. This work emphasized the form of crystal employed and brought neutrophils to the fore as central to the inflammatory response. A generation later we showed that in mixed leukocyte preparations given crystals, proinflammatory cytokines were being generated primarily by the few percent monocytes present. Mononuclear phagocytes appear to be driving the neutrophils throughout the course of the gouty paroxysm. In current phagocytic and videomicroscopic work, we suggest that the diagnostic crystals seen sometimes within neutrophils in gouty synovial fluid are the sequentially late result of inflammatory events driven by mononuclear phagocytes that have interacted with urate crystals and then with neutrophils in the synovial tissue.



We prepared MSU crystals under sterile, pyrogen-free conditions, as described previously (28). We characterized them by spectrometric analysis for triclinic morphology and examined them under conventional light microscopy and polarized light microscopy for birefringence. The crystals ranged from 5 to 20 μm in length. These crystals stimulate neutrophils effectively (calcium mobilization; tyrosine phosphorylation; phosphorylation of Erk, Akt, and p85; oxidative burst; degranulation) (17).


We collected fresh, heparinized (with sodium heparin) blood in two, 7-ml tubes from anonymous human donors and allowed them to sediment at an angle of ~60° at room temperature for ~1 h. We concentrated leukocytes from the buffy coat in a microcentrifuge (Costar, Cambridge, MA, USA; Model #8455) for ~30 s at 10,000 rpm (5585 g), washed them once in NCTC buffer (NCTC 135, Eurobio, Paris, France) and once in 10% autologous serum/NCTC (the media), and resuspended them in 100 μl media. When indicated, we separated monocytes and neutrophils by Ficoll-Hypaque sedimentation. We added 20 μl media to a precipitate of urate crystals sedimented from 20 μl of a crystal suspension of urate crystals, 5 mg/ml, yielding 0.5 mg/ml crystals in 10% autologous serum/NCTC. We deposited a drop of this suspension, sufficient only to wet an entire overlying, 22-mm × 32-mm coverslip (~4 μl) on a clean glass slide precoated with glycol methacrylate (29), sealed the preparation with paraffin, and removed it to the warmed (33°C) stage of a Zeiss phase-contrast, partially polarized photomicroscope (objective, 40× or 25×), which was connected via a Hamamatsu microscope video camera C2400 (Hamamatsu Photonics K. K., Hamamatsu City, Japan) to a Panasonic time-lapse video recorder AG6720 (Matsushita Electric Industrial, Osaka, Japan). Crystals appear white or black, depending on their orientation. Videos are 16× real time.

Crystal-leukocyte interactions

We sought incubation conditions to maximize the association of crystals and cells, including various conditions of shaking, sedimentation, and incubation times before examining the cells on a warmed stage, with actual counts of crystal-associated cells (association means “on” or “in,” not adjacent). In brief, the most important variable appears to be close apposition between cells and crystals, suspended at 37°C for 15–30 min. Shaking and centrifugation of preparations were not necessary and in fact encouraged clumping (making counts of unclumped cells unreliable). In most series, we counted >250 consecutive neutrophils and >100 monocytes. IL-1β and GM-CSF were from Sigma-Aldrich (St. Louis, MO, USA).


Paired t tests were used (two-tailed). Each n is made up of results from different donors on different days.

Supplementary Material

Supplemental Data:


Supported in part by NIH/NIAMS AR053495.


The opinions expressed in editorials, essays, letters to the editor, and other articles comprising the Up Front section are those of the authors and do not necessarily reflect the opinions of FASEB or its constituent societies. The FASEB Journal welcomes all points of view and many voices. We look forward to hearing these in the form of op-ed pieces and/or letters from its readers addressed to gro.besaf@slanruoj.


This article includes supplemental data. Please visit http://www.fasebj.org to obtain this information.


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