After severe warm I/R renal injury, a pronounced acute tubular necrosis occurs during the first 12-24 h in the absence of a marked cellular infiltrate, but with an important renal MPO activity, reflecting the activation of the adhering inflammatory cells (polymorphonuclear cells (PMNs) and mainly monocytes/macrophages). Only later at the time and site (OSOM) of regeneration a sequential accumulation of monocytes/macrophages and T cells becomes prominent, in contrast with the low number of neutrophils found in the kidney during the 10-day post-ischaemic period. The non-specificity of the so-called neutrophil-specific identification methods (MPO activity, naphthol AS-D chloroacetate esterase, or mAb HIS-48 staining), cross-reacting with monocytes/macrophages, explains the controversy in literature concerning the number of PMNs in post-ischaemic injury.

Much research has been performed to gain better insight into the regeneration process, responsible for the functional and morphological recovery after acute renal failure (ARF). Many investigators focused on endogenously produced polypeptide growth factors as the major mediators of tubular epithelial cell proliferation. However, arguments contradicting this hypothesis have recently gained more support. Indeed, the early decrease of renal epidermal growth factor (EGF) and insulinlike growth factor-1 (IGF-1) in different experimental models of ARF has been frequently shown at both the mRNA and protein level, while other growth factors could not be shown to increase. Moreover, the inaccessibility of the upregulated receptors for endogenously produced growth factors has encouraged research to seek alternative origins for the signals inducing renal regeneration. The accumulation of mononuclear leukocytes in the renal interstitium is a striking observation in renal failure. Where the interstitial disease, recognized by the persistent interstitial accumulation of leukocytes, is a better predictor of chronic renal failure and developing fibrosis, ARF distinguishes itself by the disappearance of the infiltrate when regeneration is complete. The existence of a regenerative potential provided by the network of inflammatory mononuclear leukocytes is supported by studies on tissue repair in different fields. This review discusses the infiltrating network of mononuclear leukocytes as a major participant in the regeneration process after acute renal failure, including the approach which can be followed to investigate this hypothesis.

Prevention of cellular infiltration by mAbs to LFA-1 and ICAM-1 has no effect on renal morphology, function, or regeneration following mercuric chloride-induced ARF in the rat. This result contrasts with the functional protection of the rat kidney to ischemia/reperfusion injury by virtue of an identical antibody treatment protocol. Resolving that controversy should bring better insight in fundamental processes underlying different types of ARF, and will be the subject of further study.

The transient presence of infiltrated leukocytes in the kidney during acute renal failure as well as the location of these cells within the renal interstitium suggest their association with tubular injury and/or regeneration. To date, however, neither a positive nor a negative contribution of these cells to the pathophysiology of this disease could be unambiguously demonstrated. Ill-defined methods for identifying interstitial leukocytes have added to the controversy concerning the role of inflammatory cells in renal regeneration. The current literature survey presents a qualitative description of the renal interstitial accumulation of leukocytes as observed in some acute renal failure models, with special attention to those displaying acute tubular necrosis of particular nephron subsegments. We conclude that lethal or sublethal injury to renal tubular epithelial cells following toxic or ischemic insults leads to the manifestation of an interstitial mononuclear cell infiltrate. Whereas macrophages and T lymphocytes almost invariably take part, the former being the dominant cell population with respect to both magnitude and presence over time, polymorphonuclear cells seem to be significantly increased only in the case of pyelonephritis. Infiltrating cells have often been regarded rather harmful to the tissue, mainly due to the quite well understood injuring capacity of the latter. On the other hand, we speculate mononuclear leukocytes through their potential of producing different cytokines and growth factors (FGF, TGF-α, EGF-like, IL-2, etc.) might well play an initiating and mediating role in renal regeneration after acute tubular necrosis. Therefore, the role of infiltrating leukocytes in the injury/regeneration process during acute renal failure remains highly controversial and should be further elucidated.

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