Monocytes in the rat: phenotype and function during acute allograft rejection

Cells of the monocyte/macrophage system originate from the bone marrow, reach the organs via the blood, immigrate through postcapillary venules and further differentiate into organ-specific tissue macrophages. In rats and other species, activated monocytes/macrophages aggravate autoimmune reactions, rejection of non-vascularized allografts and chronic allograft rejection. It is very likely that they also contribute to acute allograft destruction. So far it has been impossible to distinguish the function of monocytes from that of macrophages, because cell phenotypes and their alterations upon activation are ill-defined. We have thus begun to characterize the ex vivo phenotype and function of rat monocytes in the normal state and during renal allograft rejection. Monocytes are recovered from both the central and the marginal blood pool by perfusing either the recipient's circulation or the allograft vasculature. Rat monocytes have a unique surface phenotype. During allograft rejection or after infusion of interferon-gamma they up-regulate class II MHC molecules, CD161 (NKR-P1A), CD62L and CD8, while CD4 and CD43 are down-modulated. Activated perfusate monocytes exert increased in vitro cytotoxicity against tumour targets, which differs from that of NK cells. We speculate that activated monocytes contribute to kidney allograft destruction by directly damaging endothelial cells or by promoting intravascular coagulation.

Accumulating monocytes in the vasculature of rat renal allografts: phenotype, cytokine, inducible no synthase, and tissue factor mRNA expression

BACKGROUND

Necrotic patches and hemorrhagic lesions develop in the renal tissue between day 4 and day 5 after transplantation of fully allogeneic DA rat kidneys to LEW recipients. These lesions are at least in part due to destruction and obstruction of blood vessels. Damage of graft endothelial cells and blood coagulation are likely to be mediated by intravascular graft leukocytes. However, this cell population has not been thoroughly characterized before.

METHODS

We perfused untreated control kidneys, renal isografts, and allografts on day 4 after transplantation with phosphate-buffered saline/ethylenediaminetetraacetic acid to harvest leukocytes from both the blood stream as well as from the marginal intravascular pool. The mRNA expression of typical products of activated monocytes was analyzed in reverse-transcriptase polymerase chain reaction experiments. Graft monocytes were purified and their immunophenotype was investigated by flow cytometry.

RESULTS

Allograft rejection led to a 10-fold increase in the number of intravascular graft leukocytes compared to isografts. A mean number of about 100x10(6) leukocytes was harvested from a single allogeneic kidney, about 73% of these cells were monocytes and most of them displayed an activated phenotype. Compared to isografts, intravascular allograft leukocytes displayed an increased expression of tumor necrosis factor-alpha, inducible NO synthase and tissue factor.

CONCLUSIONS

Our study shows that large numbers of activated monocytes accumulate inside allograft vessels. As they express genes the products of which might damage the allograft by inducing cell death or thrombosis, we speculate that they directly participate in allograft destruction.

Monocytes in the rat

We review our methods for definition and phenotypical characterisation of normal and activated rat monocytes. To obtain a comprehensive sample of all blood monocytes including cells from the marginal pool of the blood stream, we extensively perfuse the extrapulmonary circulation with cold PBS/EDTA. Normal rat monocytes are isolated from untreated specified pathogen-free male LEW rats. In vivo activated monocytes are investigated after three days of infusion of recombinant IFN-gamma or during acute renal allograft rejection. Rat monocytes are defined by reactivity with mAbs ED1 and ED9, detecting a lysosomal membrane antigen and a member of the signal-regulatory protein family, respectively, as well as by expression of CD11b. Concomitantly rat monocytes are characterized by the absence of CD5, the absence of the B cell form of CD45R, and the absence of reactivity with mAb RP-1. The majority of the monocytes from untreated LEW rats are CD4+, CD11a(high), CD18high, CD43high, CD62-L-, CD161-, and MHC class II-. Upon stimulation of the immune system in vivo, a second monocyte population increases in number. These cells have a larger diameter and an increased granularity. They are CD4-, CD11a(int), CD18int, CD43low, CD62-L+, CD161int, and MHC class II+. Although some reagents are not yet available (e.g. antibodies against rat CD14 and CD16), rat monocytes can be defined and their state of activation can be characterized. The functionally important population of monocytes, which have already marginated, is accessible by perfusion and relatively high monocyte numbers are isolated per rat. As specified pathogen-free rats are available and numerous experimental systems involving acute or chronic inflammation have been established in rats, differentially activated monocytes may be investigated. The rat is thus a suitable experimental animal for basic research on monocytes.

Leptin does not reduce body fat content but augments cold defense abilities in thermoneutrally reared rat pups

A previous study showed that recombinant leptin markedly affects the body fat content and thermoregulatory energy expenditure of lean (+/+ and +/fa) suckling-age rats, and we wanted to find out whether leptin in doses that halved body fat of cold-reared lean pups had any effect in thermoneutrally reared lean pups. When +/+ pups were artificially reared from 4 to 16 days of age at thermoneutrality and treated as before with leptin from day 7, their total metabolic rate throughout the treatment period was only 4% higher than that of the control littermates and their final body fat content only 4% lower (both P>0.05). We conclude from comparisons of the results in +/+ pups at cold and thermoneutral conditions that leptin does not stimulate, but rather disinhibits, sympathetically mediated thermoregulatory thermogenesis.

Leptin reduces juvenile fat stores by altering the circadian cycle of energy expenditure

The recently identified hormone leptin (ob protein) secreted by white adipose tissue is widely thought to provide a feedback signal limiting fat storage by decreasing food intake. By artificially rearing leptin-treated and control littermates fed identical amounts of milk, however, we show here that lean suckling-age rats treated with recombinant murine leptin can reduce fat storage solely by increasing energy expenditure. Continuous measurements of core temperature and metabolic rate show that this increase is not uniform throughout the day but is especially prominent in the morning when rat pups usually conserve energy by entering a torpor-like state. Leptin's alleviation of hypometabolic, torpor-like states is thus not restricted to cases of impaired hormone production but seems instead to be a normal biological function independent of its effects on food intake.