Lysophosphatidic acid (LPA), an extracellular lipid mediator, exerts multiple bioactivities through activating G protein-coupled receptors. LPA receptor 3 (LPA 3 ) is a member of the endothelial differentiation gene family, which regulates differentiation and development of the circulation system. However, the relationship among the LPA receptors (LPARs) and erythropoiesis is still not clear. In this study, we found that erythroblasts expressed both LPA 1 and LPA 3 , and erythropoietic defects were observed in zLPA 3 antisense morpholino oligonucleotide-injected zebrafish embryos. In human model, our results showed that LPA enhanced the erythropoiesis in the cord blood-derived human hematopoietic stem cells (hHSCs) with erythropoietin (EPO) addition in the plasma-free culture. When hHSCs were treated with Ki16425, an antagonist of LPA 1 and LPA 3 , erythropoietic process of hHSCs was also blocked, as detected by mRNA and protein expressions of CD71 and GlyA. In the knockdown study, we further demonstrated that specific knockdown of LPA 3 , not LPA 1 , blocked the erythropoiesis. The translocation of b-catenin into the nucleus, a downstream response of LPAR activation, was blocked by Ki16425 treatment. In addition, upregulation of erythropoiesis by LPA was also blocked by quercetin, an inhibitor of the b-catenin/ T-cell factor pathway. Furthermore, the enhancement of LPA on erythropoiesis was diminished by blocking c-Junactivated kinase/signal transducer and activator of transcription and phosphatidylinositol 3-kinase/AKT activation, the downstream signaling pathways of EPO receptor, suggested that LPA might play a synergistic role with EPO to regulate erythropoietic process. In conclusion, we first reported that LPA participates in EPO-dependent erythropoiesis through activating LPA 3 .

Two models to account for an antigen-specific IgE isotypic response are proposed. Both models assume a first-tiered IgE production induced by antigen and IL-4; however, the processed IgE or Ag-IgE immune complexes stimulate T epsilon cells differently in the two models. In Model I, we propose that T epsilon cells express conventional T-cell receptors which recognize IgE isotypic determinants. Model IA proposes that IgE fragments are processed and recognized along with class II MHC molecules, and T epsilon cell preferentially act on antigen-activated IgE-committed B epsilon cells via recognition of processed membrane IgE determinants but not antigens; thus T epsilon cells are in principle capable of modulating non-antigen-specific polyclonal IgE responses. Model IB proposes that IgE function as a class-restriction determinant for nominal antigens analogous to that of class II molecules, and T epsilon cells exert stringent antigen-specific IgE isotypic responses by recognizing nominal antigens restricted to IgE. T epsilon cells thus exert antigen-specific and IgE concerted immunoregulation, and do not participate in modulating polyclonal IgE production. Model II proposes a heterotypic interaction of IgE with a cell interaction receptor (or IgE Fc receptor) on T cells. T epsilon cells modulate antigen-specific IgE isotypic responses via ligation with IgE-antigen immune complexes on B-cell surface; thus, T epsilon cells in principle contribute to polyclonal IgE responses.

A murine model for studying life-long IgE tolerance was previously developed in this laboratory by perinatal IgE injection into neonates. Herein, we demonstrated that normal and immortal CD23+ B cell lines presented processed IgE via CD23-mediated endocytic pathway and triggered perinatal IgE tolerance. The observations were as followed: (a) CD23 on normal B cells or B cell hybridomas mediated IgE-dependent perinatal IgE tolerance and total IgE deficiency; and lack of either antigen-specific IgE or total IgE did not correlate with elevated levels of autologous anti-IgE in individual mice; (b) IgE tolerance-inducing capacity of CD23+ B cell hybridomas was augmented by treatment with antigen-IgE complexes or interleukin 4, and significantly inhibited by anti-CD23 prior to IgE pulsing; (c) antigen-IgE complexes were endocytosed and degraded in acid hydrolases-containing vesicles; and IgE tolerance was abrogated by treating IgE-pulsed 17A11 at 4 degrees C or 20 degrees C followed immediately by fixation, and by treating IgE-pulsed 17A11 with metabolic inhibitors that elevated intracellular pH of the endocytic vesicles. In conclusion, this study suggested that one pivotal step of genetic control of IgE responses may be exercised at the early developmental stage of T cells of the IgE lineage, and that CD23 may facilitate capture of endogenously secreted IgE, and mediate endocytic processing and presentation of self IgE epitope(s), and thus contribute to the genesis of host IgE competence.

Among all classes of Ig, IgE exhibits the highest rate of fractional catabolism of which the site and mechanisms is not understood. We construct a panel of murine B cell hybridomas to investigate the catabolism of IgE; one of these hybridomas, 17A11, constitutively expresses high levels of type II IgE FcR (Fc epsilon RII, CD23) (Kd:1.77 nM; B max: 1.65 x 10(5], and is capable of clearing receptor-bound IgE. Receptor-mediated endocytosis of IgE ligand ensues after binding monomeric and DNP-BSA:IgE immune complexes, and the binding is inhibited by treating 17A11 with anti-CD23. IgE ligands are sequestered and are not susceptible to acid stripping from the cell surface. The internalized IgE ligands redistributed into acid hydrolase containing high density lysosomal vesicles and were degraded; metabolic inhibitors such as chloroquine and monensin that elevate intracellular pH of 17A11 also prevent entry of IgE ligand into lysosomes. These observations raise the possibility that normal Fc epsilon RII-bearing mature B cells in the circulation and lymphoid tissues may function in sequestration and catabolic turnover of IgE molecules through IgE or IL-4 up-regulated Fc epsilon RII uptake; B cell Fc epsilon RII may perform an important role in determining the short biological half-life of IgE molecules, and contributes to IgE homeostasis.