IgE class switching occurs in germinal centers

IgE antibody response is known to accompany infections with helminths. Organisms like Schistosoma mansoni, Nippostrongylus brasiliensis, Heligmosomoides polygyrus or Brugia malayi are able to induce the substantial class switching to IgE and increased levels of IgE serum antibody. IgE antibodies are also associated with allergies, asthma and generally Th2 effector responses. The other murine antibody class connected to Th2 is IgG1. Despite the importance of IgE response not all facts linked to basic IgE biology are entirely clear.  For example, it is not sure whether class-switching to IgE takes place in or outside germinal centers. Another elusive issue is the hypothetical presence of separate IgE memory population. There were some suggestions that IgE class switching needs to go through IgG1 intermediates and that IgE memory is maintained in IgG1 compartment. The recent publication uses IgE-reporter mouse strain to challenge the notion that IgE response occurs exclusively in tight connection with IgG1.

The link:
http://www.nature.com/ni/journal/v13/n4/abs/ni.2256.html

Authors use the fact that the secreted and membrane forms of antibody are translated from different mRNA splice variants. The IgE reporter strain they employ has the GFP sequence inserted downstream of exon that encodes the cytoplasmic domain of membrane IgE thus connecting the fluorescent signal with the membrane antibody version. Additionally, the IgE locus of reporter mice contains 52-amino acid region (called M1) derived from human antibody. Such approach allows detecting GFP signal as approximation of IgE membrane variant synthesis and subsequently verifying the presence of IgE with antibody against M1.

To confirm the validity of this strain for in vivo studies authors employ in vitro cultures with IgE response promoting factors (anti-CD40 and IL-4). It is established that GFP-positive population can be divided into GFPint and GFPhi subsets. The presence of soluble IgE correlates with appearance of GFPhi cells and only GFPhi fraction stains positively with anti-M1 antibody. Additionally, GFPhi cells express much more of IgE mature transcript than GFPint population which contains numerous cells positive for IgG1 presence. It is therefore concluded that only GFPhi fraction encloses IgE-switched cells.

To monitor how IgE-switched population comes into existence authors use the infection with Nippostrongylus brasiliensis. They aim to detect the germinal center (B220+IgDGL7+CD95+) and memory (B220+IgD+GL7CD38hi) compartments inside GFPhi (and thus IgE-switched) subset by flow cytometry. At the initial phase of infection all GFPhi cells bear features characteristic for germinal centers. At later stages some of them transform into memory cells. The appearance of IgE germinal center and memory populations occurs at the same time as analogous IgG1 subsets. Authors interpret these data as ruling against the possibility of IgG1 transitional stage for IgE-switched cells. Investigators also confirm the functionality of IgE memory population by adoptive transfer strategies. Additionally, the publication contains data about in vivo generation of IgE-secreting plasma cells and visualizations of germinal centers with IgE-switched cells (plus IgE memory and plasma cells) in lymph nodes of infected mice.

My first reaction after reading this paper was that of astonishment. Did they know so little about IgE response not to be sure whether there are IgE-switched cells inside germinal centers? The included data do not show anything very new or unexpected. They just extend basic facts from B cell biology for the IgE population.  Investigators show in the convincing way that IgE memory and plasma compartments derive from germinal center reaction and that memory IgE cells contribute to faster antibody response on re-infection. On the whole this is very interesting stuff which illustrates how many gaps are still in our knowledge.

Talay, O., Yan, D., Brightbill, H., Straney, E., Zhou, M., Ladi, E., Lee, W., Egen, J., Austin, C., Xu, M., & Wu, L. (2012). IgE+ memory B cells and plasma cells generated through a germinal-center pathway Nature Immunology, 13 (4), 396-404 DOI: 10.1038/ni.2256