IgE-dependent signaling as a therapeutic target for allergies

Abstract

Atopic diseases are complex, with many immunological participants, but the central element in their expression is IgE antibody. In an atopic individual, the immune system pathologically reacts to environmental substances by producing IgE, and these allergen-specific IgE antibodies confer to IgE receptor-bearing cells responsiveness to the environmental substances. Mast cells and basophils are central to the immediate hypersensitivity reaction that is mediated by IgE. In humans, there are various other immune cells, notably dendritic cells and B cells, which can also bind IgE. For mast cells, basophils and dendritic cells, the receptor that binds IgE is the high-affinity receptor, FcɛRI. For B cells and a few other cell types, the low affinity receptor, FcɛRII, provides the cell with a means to sense the presence of IgE. This overview will focus on events following activation of the high-affinity receptor because FcɛRI generates the classical immediate hypersensitivity reaction.

Figure 1

Simplified cartoon of the earliest steps in IgE/FcεRI-dependent signaling. IgE-mediated stimulation of mast cells/basophils begins with aggregation of FcεRI when multivalent substances bind to the cell-surface IgE, which is bound to FcεRI. Some form of aggregation is a requisite for activation, although there is considerable variability in how the aggregate can be formed. For example, in addition to the standard model of antigen:antibody aggregation, simply concentrating FceRI into a localized region of the cell membrane can induce activation [74]. Aggregation appears to be necessary to position the beta subunits of FcεRI so that they are trans-phosphorylated by a closely associated src-family kinase, lyn. This is step 1. The phosphorylated beta subunit of FcεRI binds lyn more tightly, allowing lyn to phosphorylate the gamma subunit of FcεRI. The phosphorylated gamma subunit recruits syk. This is step 2. Syk auto-phosphorylates and becomes more active, initiating many subsequent steps. One substrate of syk is the regulatory subunit of PI3K (phosphotidylinositol 3′ kinase) which becomes active and begins phosphorylating inositol phospholipids. This is step 3. The expanded portion of the cartoon shows just a couple of the possible forms of phosphorylated inositol phospholipids. These particular forms are regulated by multiple enzymes including PI3K, SHIP (SH2-containing inositol-5′-phosphatase) and PTEN (phosphatase and tensin homolog deleted in chromosome 10) the latter of which simply reverses the effects of PI3K. Activity of PTEN appears constitutive but controls the set-point of these reactions. The phosphotidyl-3,4,5-trisphosphate (PIP3) in the plasma membrane acts as a recruitment site for other enzymes including btk (which binds through a PH domain). This is step 4. Btk is activated through phosphorylation, probably by a src-family kinase (not shown). Each of these enzymes in steps 1–4 mediate multiple downstream signaling steps that are critical for initiation and maintenance of the secretory response in mast cells/basophils. Inhibition of each step can completely ablate IgE-mediated functions, although inhibition of lyn needs to be nearly complete to see suppression due to its role in initiating termination steps.

Figure 2

Termination steps during IgE-mediated activation. The cartoon expands on the behavior of SHIP presented in figure 1. This multi-domain enzyme is capable of regulating the presence of PIP3 which recruits various activation enzymes to the plasma membrane. But at the C-terminal end of SHIP, near the proline rich domain (PRD), a tyrosine phosphorylated by a src-family kinase, can recruit the adaptor protein dok (downstream of kinase) for which there are several forms. Dok recruits a rasGAP protein which down-regulates the activity of p21 ras, another small G-protein that is crucial in the initiation of the MAPK (mitogen-activated protein kinases) pathway and other pathways to secretion. The MAPK kinase pathway (raf->MEK->Erk) specifically regulates cPLA2 and the arachidonic acid metabolism pathway but p21Ras controls other pathways as well. On the right side of the cartoon is a representation of the potential role of SHP-1 (SH2-containing tyrosine phosphatase). SHP-1 represents a family of tyrosine phosphatases that are actively recruited into activation cascades, but there are also constitutively acting tyrosine phosphatases that control set-points for activation. The cartoon represents only a small subset of the negative feedback loops that operate during activation, but these two phosphatases act as negative feedback on early steps of the cascade.

Figure 3

Therapeutic approaches to down-regulating the presence of IgE. Panel A shows the schema for the sole current IgE-reducing approach. Omalizumab is an anti-IgE antibody that binds to an epitope on IgE that is necessary for interaction with FcεRI prevents IgE from binding to FcεRI. Because the specific epitope is sterically inaccessible when IgE is bound to the receptor, this approach should not result in direct activation of mast cells/basophils. However, the approach results in large amounts of circulating total IgE because it does not influence the synthesis of IgE. The current therapeutic, omalizumab, has an affinity that is not equivalent to the natural affinity of IgE for FcεRI. Consequently, high quantities of omalizumab are needed for treatment to effectively reduce free IgE levels. But this process is assisted by the natural biology of FcεRI expression control. An unoccupied cell surface FcεRI is not stable and leaves the cell surface with a half-life (in vitro) of 1 day. Consequently, reductions in free IgE also result in reductions in the density of FcεRI on mast cells/basophils. This effect is a double-edged sword because slight increases in free IgE result in more dramatic increases in cell surface IgE than would expected on the basis of equilibrium binding because FcεRI expression increases. However, higher affinity anti-IgE antibodies working by the same mechanism may solve many of the problems associated with omalizumab. Panels B and C illustrate two methods to block the transition of B cells to IgE-secreting plasma cells. Panel B; the illustration shows a hybrid approach in which an anti-IgE antibody is administered that accomplishes the goals of omalizumab but also suppresses IgE synthesis. It does this by engineering the Fc portion of the therapeutic antibody to bind with high affinity to CD32b on B cells. Co-crosslinking the CD32b with the plasma membrane IgE that is exclusively on B cells that have class switched, (i.e. it does not affect B cells destined for synthesis of other antibody classes) suppresses the progression of the B cells towards plasma cells. Panel C; the cartoon depicts a third approach. Fortuitously, membrane-associated IgE includes a short peptide sequence near the transmembrane segment (labeled CemX) that is not found on solution phase IgE. Antibodies to this short sequence target B cells destined to make plasma cells that secrete IgE and crosslinking the membrane IgE induces suppression of the B cell-to-plasma cell transition.