www.sciencemag.org SCIENCE VOL 343 28 FEBRUARY 2014 983
distal signals are generated and genes are
Two Src kinases are thought to initiate
FcεRI signaling. Fyn phosphorylates the
cytosolic adapter Gab2, enabling the recruitment of phosphatidylinositol 3-kinase (4).
Lyn phosphorylates FcRβ and FcRγ ITAMs,
enabling the recruitment of the tyrosine
kinase Syk. Syk subsequently phosphorylates the many tyrosines of the transmembrane adapters LAT1 and LAT2 that nucleate the FcεRI signalosome. Although LAT1
(5) and LAT2 (6) each generate a mixture of
positive and negative signals, the two adapters play antagonistic roles: LAT1 positively
regulates, whereas LAT2 negatively regulates
FcεRI signaling (7). The molecular basis of
this antagonism is poorly understood.
Suzuki et al. compared the effects of a
high-affinity antigen [dinitrophenyl-capro-
ate-Fab (DNP)] and of a low-affinity antigen
[2-dinitrophenyl-caproate-Fab (2NP)] that
are recognized by the same IgE, at concen-
trations that induce comparable FcεRI phos-
phorylation in cultured mouse mast cells. As
expected, DNP triggered more robust Lyn-
Syk-LAT1–dependent signals than 2NP, lead-
ing to an increase in the release of a granular
enzyme (β-hexosaminidase), the production
of a proinflammatory lipid (leukotriene C4),
and the secretion of cytokines (tumor necrosis
factor and interleukins 6 and 13). By contrast,
2NP induced more chemokine secretion [che-
mokine motif ligand (CCL) 2, 3, and 4]. Unex-
pectedly, LAT2 was more phosphorylated
upon stimulation by 2NP than by DNP, and
the adapter colocalized with FcεRI. Another
Src kinase, Fgr, colocalized with FcεRI and
could enhance LAT2 phosphorylation. Che-
mokine secretion was impaired by either Fgr
or LAT2 deficiency, whereas degranulation
was increased. DNP induced passive cuta-
neous anaphylaxis (a reaction characterized
by increased permeability of blood vessels
in the skin) of a greater magnitude than did
2NP. Infiltration of the dermis that follows
passive cutaneous anaphylaxis was enriched
in neutrophils in response to DNP, but was
enriched with monocytes and macrophages in
response to 2NP. Thus, the Src kinases that initiate FcεRI signaling differentially control the
use of LAT1 and LAT2, which in turn determines the relative abundance of cytokines
and chemokines that are produced. This ultimately regulates the cell types that contribute
to inflammatory reactions.
Early and late signals triggered by FcεRI
are tightly controlled. Early signals depend
on antigen affinity through kinetic proofreading. Late signals are controlled by the
lipid phosphatase SHIP1 (8). How SHIP1 is
recruited into FcεRI signalosomes is unclear,
but SHIP1 phosphorylation (a consequence
of its membrane recruitment) increases with
the concentration of antigen (9). SHIP1-
dependent negative regulation therefore
depends on mechanisms that sense the extent
of FcεRI aggregation. Suzuki et al. unravel
an additional mechanism that controls the
quality of mast cell responses. Thus, distinct
mechanisms sense the concentration and
the affinity of antigen and concur to control
FcεRI-dependent mast cell activation both
quantitatively and qualitatively.
FcεRI resembles antigen receptors
expressed by B and T cells, whose signaling
depends on the affinity of their interaction
with antigen. Kinetic proofreading may in
part account for differences in the magnitude
and in the quality of T (10) and B (11) cell
responses. No LAT1-LAT2 shift can occur
in lymphocytes that express one adapter only
(LAT1 in T cells, LAT2 in B cells). Many
other cells besides mast cells, including natu-
ral killer cells, macrophages, neutrophils, and
platelets, express both adapters. All express
receptors for the Fc portion of IgG (FcγR)
that use the same ITAM-dependent signaling
as FcεRI when engaged by antigen-antibody
complexes. Like mast cells, they have a wide
functional repertoire and they contribute to
both protective immunity and inflammatory
diseases. One expects the LAT1-LAT2 bal-
ance to also control FcγR signaling in these
cells. The identification of mechanisms that
affect this balance is a key to understand-
ing the protective and pathogenic effects
of immune responses, and consequently
to designing new therapeutic tools with
increased efficacy and decreased toxicity.
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Receptor discrimination. Allergy-evoking antigens
(Ag), depending on their affinities for IgE antibodies, can elicit different mast cell responses by coupling the IgE receptor (FcεRI) with different Src family kinases (Lyn and/or Fgr). This results in differential
use of the adapter proteins LAT1 and LAT2, which promote preferentially cytokine- or chemokine-driven
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