Origin of Macrophages

The zebrafish lysozyme C promoter drives myeloid-specific expression in transgenic fish

Background

How different immune cell compartments contribute to a successful immune response is central to fully understanding the mechanisms behind normal processes such as tissue repair and the pathology of inflammatory diseases. However, the ability to observe and characterize such interactions, in real-time, within a living vertebrate has proved elusive. Recently, the zebrafish has been exploited to model aspects of human disease and to study specific immune cell compartments using fluorescent reporter transgenic lines. A number of blood-specific lines have provided a means to exploit the exquisite optical clarity that this vertebrate system offers and provide a level of insight into dynamic inflammatory processes previously unavailable.

Results

We used regulatory regions of the zebrafish lysozyme C (lysC) gene to drive enhanced green fluorescent protein (EGFP) and DsRED2 expression in a manner that completely recapitulated the endogenous expression profile of lysC. Labeled cells were shown by co-expression studies and FACS analysis to represent a subset of macrophages and likely also granulocytes. Functional assays within transgenic larvae proved that these marked cells possess hallmark traits of myelomonocytic cells, including the ability to migrate to inflammatory sources and phagocytose bacteria.

Conclusion

These reporter lines will have utility in dissecting the genetic determinants of commitment to the myeloid lineage and in further defining how lysozyme-expressing cells participate during inflammation.

Amphibia Kupffer cells

Amphibia Kupffer cells (i.e., liver resident macrophages) show many common characteristics when compared with Mammalia Kupffer cells: filopodia, microvillous-like structures, lamellipodia, fuzzy coat, coated vesicles, bristled vacuoles, nonspecific esterase activity, and pinocytotic and phagocytic activity are present both in Amphibia and Mammalia Kupffer cells. On the other hand, some differences are present between Kupffer cells of both zoological classes: phagocytosed red cells and their derivatives, iron-protein complexes, and lipofuscin bodies are normally present in Amphibia Kupffer cells, but absent in the same cells of healthy mammals. Worm-like structures are not seen in Amphibia and endogenous peroxidase activity is very weak in these animals compared with Mammalia. The most important difference lies in the ability of Amphibia Kupffer cells to produce melanins: in fact the tyrosinase gene is expressed, "melanosome centers" are present, and dopa oxidase activity is demonstrable.

Proliferation of mature and immature subpopulations of bronchoalveolar monocytes/macrophages and peripheral blood monocytes

A continuous influx of peripheral blood monocytes (PBM) to the lung is thought to maintain the local population of alveolar macrophages (AM). However, local proliferation of a small subpopulation of AM has been demonstrated in animal studies and in humans. AM exhibit a great heterogeneity with regard to their morphology (cell size, shape of nucleus), immunophenotype (expression of CD14 and RFD9 antigen), and function. Part of this heterogeneity may be explained by the presence of different maturation stages of AM, ranging from small immature, CD14+ RFD9- PBM-like cells to large, CD14- RFD9+ mature AM. These findings prompted us to study whether proliferation of PBM and AM is related to their stage of maturation. The expression of the proliferation marker Ki-67 was studied in AM from both healthy volunteers and patients suffering from sarcoidosis. Using double immunofluorescence staining, we studied proliferation of immature, CD14+ AM, and mature, RFD9+ AM in sarcoidosis, and we compared this with PBM. A significantly larger percentage of AM in general expressed Ki-67 antigen in sarcoidosis (3.0 (median); range 1.1-5.5) as compared with healthy volunteers (0.8; 0.2-1.3). In sarcoidosis, proliferation was observed in both the immature and the mature subpopulation of AM. Proliferating PBM were rarely observed [less than 0.2% of the CD14+ mononuclear cells (MNC)] both in healthy volunteers and sarcoidosis patients. A small subpopulation of PBM showed a weak expression of RFD9 antigen (less than 1% of MNC). Interestingly, proliferation of PBM was concentrated in this subpopulation (15% of the RFD9+ MNC). These data show that even mature AM, which are generally thought to be terminally differentiated cells with little capacity to replicate, are able to proliferate, whereas a relatively very low percentage of their precursors in the blood circulation proliferates. Furthermore, the findings suggest that lung tissue in sarcoidosis creates an environment which promotes proliferation of monocytic cells. Pulmonary alveolar macrophages (AM) were originally recognized as phagocytosing scavenger cells (Ham & Cormack 1979), but presently they are also known to initiate and regulate inflammatory and immunological processes in several lung diseases (Herscowitz 1985, Unanue & Allen 1987, Sibille & Reynolds 1990). AM are thought to represent more mature cells of the mononuclear phagocyte system, and to be derived from peripheral blood monocytes (PBM) (Van Furth 1982, Ginsel 1993). As AM are continuously lost (mainly through a transport from the peripheral airways, via the trachea to the pharynx), the local AM population must be constantly replenished.(ABSTRACT TRUNCATED AT 400 WORDS)