Antigen presentation — a transformed approach

Abstract

Listen

Translate article

Cloned lines of macrophages in differing states of matura- tion or activation are potentially useful tools in in-vitro investigations of both the afferent and efferent limbs of immune responses. In particular, the availability of macrophage-like lines should help to clarify how mono- nuclear phagocytes present antigen to T lymphocytes, since through cloning techniques it is possible to obtain lines uncontaminated by dendritic cells and B cells, the other major cell types which present antigen 1'2. To date, the results of attempts to generate lines of this type have been unconvincing: in the mouse, at least, few macro- phage-like cell lines have been produced which express the Ia antigens necessary for antigen presentation s. This level of success may be low because, unlike dendritic and B cells, resident mouse peritoneal macrophages do not constitutively express Ia antigens unless activated by lymphokinC or ),-interferon (R. A. B. Ezekowitz et al., unpublished observations). These agents also induce certain cell lines to express Ia molecules 5. Although future studies with such 'induced' cell lines will aid our understanding of the accessory function of mononuclear phagocytes, it should be borne in mind that many of the currently available cell lines are defective in one or more functions characteristic of untransformed macrophages. This may be due to the establishment of genetic variants during long-term culture or in some cases to somatic mutations arising from the transformation event itself. Such variants are useful in studies of somatic cell genetics 6 but their presence in investigations of a normal physiological function such as antigen presenta- tion may give rise to spurious or misleading results. What is needed, therefore, is a reliable method of producing macrophage-like cell lines which, besides expressing Ia antigens, retain all the functional properties of the parental cells. A significant step forward in this direction has now been taken by Yumiko Nagata and colleagues in New York who, for the first time, have transformed human monocytes into macrophage-like cell lines bearing HLA- DR and DS determinants 7. Until now SV40 virus could not be used to generate human macrophage-like lines (as it is with mouse cells) because the virus lysed the cells. This problem was overcome by the use of origin-defective SV40 DNA, cloned in Escherichia coli, to transfect human monocytes by calcium phosphate precipitation. After I-2 months the New York group obtained three independent lines which possessed the expected characteristics of mononuclear phagocytes such as phagocytosis, lysozyme secretion, expression of Fc and complement receptors. Moreover, two of these cell lines were able to stimulate a mixed lymphocyte reaction which could be inhibited by prior incubation with anti-DR antibody. The DR and DS determinants consistently found on the human macro- phage lines probably reflect the parental phenotype since human monocytes, unlike mouse resident peritoneal macrophages, constitutively express these antigens 8. From these results it can be anticipated that successful transformation of )'-interferon-treated mouse macro- phages would give rise to cell lines expressing Ia antigens. While this is an exciting possibility it may not be easy to accomplish because successful transformation with tumorigenic viruses depends on at least one cycle of DNA replication. With human monocytes this was achieved by prior incubation with a macrophage growth factor to which the monocytes could respond; whether Ia-bearing, interferon-treated mouse macrophages behave in a similar way awaits investigation. Another intriguing possibility is the transformation of dendritic cells into lines which retain their functional pro- perties. Ia-bearing dendritic-like cell lines have been describe&, but how these relate to the normal splenic population is unclear since they were derived from variants of a mouse leukaemia cell line, P338. The dendritic cells of the spleen replenish themselves at the rate of 10 % per day1°, so transformation of these cells may be a less formidable task than transformation of activated macrophages. Clearly, the identification of a dendritic cell growth factor would be helpful in this respect. To date, none has been described. Alternatively, it may be possible to derive dendritic cell lines from their precursors in the bone marrow, perhaps by using suitable retroviral transforming agents followed by selection of lines bearing the dendritic cell-specific marker 33D111. While the feasi- bility of these approaches is currently unknown, the avail- ability of cloned lines of antigen-presenting cells would clearly be of great value in our understanding of how antigen-specific immune function is stimulated. References