Birgitta Heyman - 45 years of antibody feedback regulation
Antibodies can regulate the antibody response against the antigen they bind to. This phenomenon is called antibody feedback regulation and can be both positive, leading to more than 100-fold enhancement, or negative, resulting in almost complete suppression. My research has been aimed at understanding the molecular mechanisms behind these regulatory circuits.
In 1978 I started as a PhD student with professor Hans Wigzell as supervisor at the Department of Immunology at Uppsala University. By then I had realized that working with patients was not my cup of tea and that research would be more fun. Hans presented me with a paper by Niels Jerne and Claudia Henry from 1968 showing that IgM anti-SRBC (sheep red blood cells), passively administered together with SRBC to mice, enhanced the antibody response against SRBC. In the same situation, IgG anti-SRBC almost completely suppressed the antibody response. My PhD project was to elucidate the mechanisms behind these dramatic immunoregulatory effects of IgM and IgG and this area has continued to fascinate me throughout my career.
With the exception of a postdoc period at Scripps Clinic and Research Foundation, La Jolla, California in William Weigle's lab (1983-84) and a visiting professorship at Harvard, Boston in Michael Carroll's lab (2001-02), I have been employed by Uppsala University, albeit at several different departments: Department of Immunology, Department of Medical Chemistry, Department of Pathology, Department of Genetics and Pathology (where I became professor of Experimental Pathology in 1999 and professor of Experimental Immunology in 2002). In 2008, my group moved to Department of Medical Biochemistry and Microbiology where I am now professor emerita.
IgG-mediated suppression. The suppressive effect of IgG takes place whether or not the IgG antibodies can activate complement or bind to Fc-receptors. This, together with many other experimental findings, suggests that suppression is in fact caused by mere binding of IgG to the antigen, thereby hiding it from recognition by the immune system. IgG-mediated suppression is used clinically to prevent haemolytic disease of the newborn, which can occur in Rhesus positive fetuses carried by Rhesus negative women. Whether the mechanism above applies also to human Rhesus prophylaxis is currently a matter of debate but I find it unlikely that they should be unrelated phenomena.
IgM-mediated enhancement. The other part of my PhD project was to explain why IgM antibodies enhanced the antibody response. After returning from Scripps and having started my own lab, we could show that mutant monoclonal IgM antibodies which had lost their ability to activate complement also lost their enhancing ability. Much later, in collaboration with Michael Carroll, we constructed a knock-in mouse (Cm13) with the same point mutation as the mutant IgM antibody. This mouse had a slightly reduced antibody response showing that endogenous IgM, via complement, indeed plays a role in upregulating antibody responses. However, the reduction was far from as pronounced as that seen in complement deficient mice and thus cannot fully explain the role of complement in antibody responses. The most likely explanation for IgM-mediated enhancement is that antigen-IgM-complement immune complexes are efficiently transported to areas in the spleen where immune cells interact with antigen to initiate antibody responses.
IgE-mediated enhancement. In 1993, we found that IgE, passively administered together with protein antigens, could enhance antibody responses, sometimes more than a 100-fold. In a series of papers, the last one published in 2016, this multi-step process was investigated. It turns out that IgE-antigen complexes after immunization are rapidly captured by B-cells circulating in the blood and expressing an IgE-receptor, CD23. Within 30 minutes, the IgE-complexed antigen (but not uncomplexed antigen) can be found in the B cell areas of the spleen and is then transferred to a subtype of dendritic cells. These endocytose and present peptides to specific CD4+ T helper cells which increase dramatically in numbers during the first 3 days. The efficient help given by these T cells to antigen specific B cells most likely explains the enhanced antibody responses observed.
IgG-mediated enhancement. A fourth branch of our feedback regulation studies concerns the ability of IgG antibodies, administered together with protein antigens, to enhance antibody responses. One of the murine IgG subclasses, IgG3, executes this through the same mechanism as IgM, i. e. antigen-IgG3-complement complexes are being formed and transported to the "right" areas for immunostimulation in the spleen. Interestingly, the other IgG subclasses do not rely on complement for their enhancing effects but instead use Fc-receptors for IgG. Most likely, the antigen-IgG complexes bind to Fc-receptors on dendritic cells which endocytose them, and present peptides to CD4+ T helper cells, subsequently offering efficient help to antigen specific B cells.
In summary, feedback regulatory effects of antibodies are often caused by direct interference with the antigen. Antigen can be hidden, it can be captured for transport to optimal areas for immune cell stimulation, or it can be endocytosed and presented efficiently to T helper cells. We have found little evidence for direct B cell regulation through inhibitory IgG Fc-receptors or stimulatory complement receptors expressed on the B cells themselves.