Granulomas are immune cell aggregates formed in response to persistent inflammatory stimuli. Granuloma macrophage subsets are diverse and carry varying copy numbers of their genomic information. The molecular programs that control the differentiation of such macrophage populations in response to a chronic stimulus, though critical for disease outcome, have not been defined. Here, we delineate a macrophage differentiation pathway by which a persistent Toll-like receptor (TLR) 2 signal instructs polyploid macrophage fate by inducing replication stress and activating the DNA damage response. Polyploid granuloma-resident macrophages formed via modified cell divisions and mitotic defects and not, as previously thought, by cell-to-cell fusion. TLR2 signaling promoted macrophage polyploidy and suppressed genomic instability by regulating Myc and ATR. We propose that, in the presence of persistent inflammatory stimuli, pathways previously linked to oncogene-initiated carcinogenesis instruct a long-lived granuloma-resident macrophage differentiation program that regulates granulomatous tissue remodeling.

Immunity to mycobacteria involves the formation of granulomas, characterized by a unique macrophage (MΦ) species, so-called multinucleated giant cells (MGC). It remains unresolved whether MGC are beneficial to the host, that is, by prevention of bacterial spread, or whether they promote mycobacterial persistence. Here, we show that the prototypical antimycobacterial molecule nitric oxide (NO), which is produced by MGC in excessive amounts, is a double-edged sword. Next to its antibacterial capacity, NO propagates the transformation of MΦ into MGC, which are relatively permissive for mycobacterial persistence. The mechanism underlying MGC formation involves NO-induced DNA damage and impairment of p53 function. Moreover, MGC have an unsurpassed potential to engulf mycobacteria-infected apoptotic cells, which adds a further burden to their antimycobacterial capacity. Accordingly, mycobacteria take paradoxical advantage of antimicrobial cellular efforts by driving effector MΦ into a permissive MGC state.

Background-The intriguing monotony in the occurrence of intercaval conduction block during typical atrial flutter suggests an anatomic or electrophysiological predisposition for conduction abnormalities. Methods and Results-To determine the location of and potential electrophysiological basis for conduction block in the terminal crest region, a high-density patch electrode (10ϫ10 bipoles) was placed on the terminal crest and on the adjacent pectinate muscle region in 10 healthy foxhounds. With a multiplexer mapping system, local activation patterns were reconstructed during constant pacing (S 1 S 1 ϭ200 ms) and introduction of up to 2 extrastimuli (S 2 , S 3 ). Furthermore, effective refractory periods were determined across the patch. If evident through online analysis, the epicardial location of conduction block was marked for postmortem verification of its endocardial projection. Marked directional differences in activation were found in the terminal crest region, with fast conduction parallel to and slow conduction perpendicular to the intercaval axis (1.1Ϯ0.4 versus 0.5Ϯ0.2 m/s, PϽ0.01). In the pectinate muscle region, however, conduction velocities were similar in both directions (0.5Ϯ0.3 versus 0.6Ϯ0.2 m/s, PϭNS). Refractory patterns were relatively homogeneous in both regions, with local refractory gradients not Ͼ30 ms. During S 3 stimulation, conduction block parallel to the terminal crest was inducible in 40% of the dogs compared with 0% in the pectinate muscle region. Conclusions-Even in normal hearts, inducible intercaval block is a relatively common finding. Anisotropic conduction properties would not explain conduction block parallel to the intercaval axis in the terminal crest region, and obviously, refractory gradients do not seem to play a role either. Thus, the change in fiber direction associated with the terminal crest/pectinate muscle junction might form the anatomic/electrophysiological basis for intercaval conduction block.

Recent in vitro studies have described regional differences of ion current expression and function, possibly accounting for reduced homogeneity of repolarization in the heart. In 11 intact canine hearts regional disparity of repolarization was determined at baseline and after administration of the I(Kr) blocking agent dofetilide (30 microg/kg) and the I(Ks)-blocking agent chromanol 293b (10 mg/kg). Effective refractory periods (ERPs) were determined through up to 10 needle electrodes inserted into basal, midwall and apical regions of the left ventricular wall using the extrastimulus technique (cycle length [CL] 300 and 850 ms). At baseline (CL of 850 ms), ERPs were significantly longer in epicardial muscle layers of the apex compared to the base. In deeper muscle layers regional differences of ERPs were not detectable. Administration of dofetilide increased apico-basal disparity of repolarization, due to a more marked increase of ERPs in the apex than in the base. In contrast, homogeneous ERPs were evident along the apico-basal axis after administration of chromanol 293b. Transmural dispersion of refractoriness could not be observed in any region at baseline, or after drug-administration. In the intact canine heart, apico-basal disparity of repolarization varies between individual muscle layers. Dependent on their current specificity, antiarrhythmic agents may enhance or diminish regional disparity of repolarization.

Multisite pacing can prevent functional conduction blocks by multidirectional excitation and a reduction in total activation time. Triple-site and, possibly, septal pacing modes are expected to be most efficient because both minimize total activation times and maximize the multidirectionality of excitation. In spite of unaffected local refractory periods, the shortening of local recovery intervals might homogenize atrial repolarization and, thus, contribute to the preventive effects of multisite pacing.

At the heart rates applied, the in vivo canine heart does not exhibit regional differences in electrophysiological properties. Given the homogeneity of antiarrhythmic drug effects, induction of local gradients of refractoriness is obviously not a common mechanism of proarrhythmia in normal hearts.

Myocardial hypertrophy associated with CAVB predisposes the canine heart to drug induced PVTs. This seems to be primarily linked to prolonged repolarization. PVTs in this model are not only initiated, but also perpetuated by a centrifugal spread of activation.

Although there is a dose-dependence as to the sustenance of mono- or polymorphic tachycardias, this does not reflect on the three-dimensional activation pattern of cesium induced arrhythmias, which are due to mono- or multifocal activation originating from the subendocardium.