Immune response: Oxysterol takes Toll on IgA

Lipidomics Gateway (27 January 2010) [doi:10.1038/lipidmaps.2010.1]

Lipidomic study reveals that secretion of 25-hydroxycholesterol by macrophages regulates the adaptive immune response.

A macrophage and the structure of 25-hydroxycholesterol.

White blood cells at mucosal surfaces protect against constant microbe exposure while limiting potentially damaging inflammation. Critical to this balance is the interplay between the innate immune system, including macrophages, and the adaptive immune response, which includes B cell activation. The main class of immunoglobulin produced by B cells in mucosa is IgA, but it is not well understood how the switch to IgA synthesis is regulated. Using a lipidomic approach developed as part of the NIH-funded LIPID MAPS consortium, David Russell and colleagues now identify an unexpected regulatory role for the bioactive lipid 25-hydroxycholesterol. Reporting in Proceedings of the National Academy of Sciences of the USA, they show that activation of a macrophage toll-like receptor (TLR) by microbial antigens induces synthesis of 25-hydroxycholesterol, and that this oxysterol potently suppresses IgA production.

Macrophage TLR4 is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria, and by its synthetic equivalent Kdo2-Lipid A (KDO). As part of the LIPID MAPS effort to define the macrophage lipidome and its response to immune challenge, the authors stimulated mouse macrophages with KDO and monitored the production of sterols using mass spectrometry. The level of 25-hydroxycholesterol in the cells and medium was substantially increased within 24 hours of KDO treatment. The expression of cholesterol 25-hydroxylase (CH25H), which catalyses production of 25-hydroxycholesterol, was likewise induced in the macrophages. CH25H messenger RNA levels were also increased in vivo by injection of KDO into mice.

To probe the effects of CH25H expression, the authors conducted microarray analysis on tissues from Ch25h^-/- mice and discovered a link with the production of IgA. Two messenger RNAs required for IgA synthesis were markedly upregulated by CH25H knockout, and an assay detected 3–5-fold increases in IgA. Similarly, IgA levels were significantly reduced by knockout of the enzyme responsible for metabolism of 25-hydroxycholesterol. No differences were found in the total number or ratio of leukocyte subpopulations, or in cytokine production, between wildtype and Ch25h^-/- mice, indicating that these factors do not control the observed changes to IgA expression.

Naïve B cells secrete IgM, and the change to IgA synthesis on activation is mediated by class switch recombination (CSR), which involves genetic rearrangement of the Ig heavy chain locus. B cells isolated from mice were treated with cytokines known to induce CSR, and the IgA levels monitored over 6 days. When added to the medium before the 4^th day, 25-hydroxycholesterol suppressed the cytokine-induced secretion of IgA, as measured on day 6. The timing indicates that 25-hydroxycholesterol has no effect on IgA secretion after CSR has occurred.

Two mechanisms were identified by which 25-hydroxycholesterol might regulate IgA CSR. Firstly, the oxysterol counteracted the proliferative effect of interleukin-2 on B cells – B cell division is required for successful CSR. However, it had no effect on proliferation stimulated by other factors, including LPS. In cells stimulated with these other agonists, 25-hydroxycholesterol instead suppressed the induction of a DNA-editing enzyme required for CSR.

The unanticipated suppression of IgA by macrophage-derived 25-hydroxycholesterol is an example of how lipidomics can rapidly identify novel links between bioactive lipids and biological processes. Further work will help to define this interaction between the innate and adaptive immune responses, and could be relevant to the generation of mucosal vaccines.

Emma Leah