A study conducted to observe the self-renewal of embryonic stem cells (ESCs) concluded that ESCs can self-renew and differentiate into all cell lineages. Calcium is a universal second messenger which regulates a number of cellular pathways. Store-operated calcium channels (SOCCs), but not voltage-operated calcium channels, are present in mouse ESCs (mESCs). In this study, store-operated calcium entry (SOCE) was found to exist in mESCs. It was observed that 17β-estradiol (E2) enhanced mESC proliferation. Pluripotent markers, Sox-2, Klf-4, and Nanog were down-regulated by 2-aminoethoxydiphenyl borate (2-APB). 2-APB is a reliable blocker of store-operated Ca2+ entry but an inconsistent inhibitor of InsP3-induced Ca2+ release, suggesting that the self-renewal property of mESCs relies on SOCE. SOCC blockers can reverse both stimulated proliferation and increased SOCE, suggesting that E2 mediates its stimulatory effect on proliferation via enhancing SOCE [1].

Naturally occurring CD4+CD25+ regulatory T cells play an important role in mediating maternal tolerance to the fetus during pregnancy, and this effect might be regulated via maternal estrogen secretion. Although estrogen concentration in the pharmaceutical range has been shown to drive the expansion of CD4+CD25+ regulatory T cells; estrogen at physiological doses, in addition to expanding T regulatory cells in different tissues, also enhances expression of the Foxp3 gene (a hallmark for CD4+CD25+ T regulatory cell function) and the IL-10 gene. Such converted CD4+CD25+ T cells have a similar regulatory function as naturally occurring regulatory T cells, as demonstrated by their ability to suppress naive T cell proliferation in a mixed lymphocyte reaction. The estrogen receptor is present in the CD4+CD25- T cells and the conversion of CD4+CD25- T cells into CD4+CD25+ T cells stimulated by estrogen could be inhibited by ICI 182,780, a specific inhibitor of ERs (endoplasmic reticulum) [2]. It is generally accepted that the conventional mature follicular and marginal zone B cell subsets are derived from the transitional B cell pool. There is some evidence for a branch position at the late transitional stage toward pre-follicular and pre-marginal zone B cells; however, the presence of such precursors requires further investigation [2]. Studies in which the expression of specific molecules of the BCR signaling cascade is altered have clearly shown that the strength of the BCR signal helps determine the fate of B cell maturation [3]. Manipulation of the BCR to produce very potent signals favors the development of the non-conventional B cell population (which arises through mechanisms distinct from the conventional B cell subsets) [4]. On the other hand, a modest decrease in signaling strength favors the development of follicular B cells. Finally, a strongly diminished BCR signal favors the development of marginal zone B cells [5]. Thus, a general paradigm has emerged in which a strong BCR signal favors B1 B cells (the cells with no memory and that undergo self-renewal in the periphery), a strong-to-intermediate signal favors follicular B cells, and a weak signal favors marginal zone B cells. However, there are some exceptions to this model. For example, mice deficient in the phosphatase and tensin homolog PTEN, which down-regulates CD19-mediated activation of phosphoinositide 3-kinase, exhibit an expansion of both marginal zone and B1 B cells [6]. There is experimental evidence that the surface density of the BCR itself can alter the development of B cell subsets [7]. Also, the specificity of the BCR may skew the development of mature subsets. According to one study, B cells of transgenic mice that expressed an anti-phosphorylcholine Ab showed preferentially development into marginal zone and B1 B cells (B cells with no memory) [8]. Although, it can be argued that the degree of BCR cross-linking, and not Ag specificity, is responsible for altering of the naive repertoire to distinct B cell subsets [9].

An auto-reactive BCR in the presence of self-Ag (antigens) is barred from entry into the mature, immune-competent B cell repertoire. This is accomplished by three distinct mechanisms of tolerance induction: receptor editing, deletion, or anergy induction. BCR engagement of immature B cells can result in the reactivation of the recombinase machinery that triggers a new specificity for B cells that is not auto-reactive [10]. B cells that successfully undergo receptor editing may be spared from elimination [11]. Immature B cells will be deleted via apoptosis if the auto-reactive specificity is not extinguished [12]. Auto-reactive B cells with less extensive BCR cross-linking are rendered anergic [13]. These cells do not undergo rapid cell death but are non-responsive to BCR engagement. The exact mechanisms by which the BCR determines the mode of negative selection have yet to be clearly established. It still remains to be determined how BCR ligation can induce negative selection of immature B cells and also activation of mature B cells. No major difference in the molecules of the BCR signal transduction pathway has been observed in the comparison of anti-IgM-treated immature and mature B cells in vitro. However, subtle differences such as increased calcium immobilization, an increase in the breakdown of inositol 1,4,5-trisphosphate, and enhanced phosphorylation of regulatory tyrosine residues have been reported and may be responsible for determining whether an apoptotic or activation program is initiated [14]. In addition, the BCR of mature B cells enters into lipid rafts following Ag engagement, whereas the BCR of immature B cells is excluded [15]. Other differences between immature and mature B cells that may affect the outcome of BCR signaling include decreased levels of the negative regulator CD22 and increased levels of the positive regulators, B cell linker protein, Btk, and phospholipase Cγ, in immature B cells [14]. There are several reports demonstrating the sensitivity of transitional type 1 B (T1 B) cells in vitro to BCR-induced apoptosis, whereas T2 B cells (transitional type 2 B cells) are resistant to it [16]. Other studies suggest that both T1 and T2 B cells are sensitive to BCR-mediated apoptosis, but only T2 B cells can be rescued by T cell help (CD40 and IL-4) [17]. The ability of T3 B cells (transitional type 3 B cells) to undergo BCR-mediated apoptosis remains to be determined. There is now increasing evidence that transitional B cells of the spleen are also the target of negative selection [18]. Not all auto-reactive B cells are eliminated from the pre-immune inventory. Cells with minimal interaction with self-antigens are ignored either because of the low-affinity of the BCR or the low concentration of accessible antigen at the immature and transitional B cell stages. These remain viable in the periphery. Once these low-affinity B cells become activated, they undergo a process of somatic mutation and affinity maturation to become high-affinity B cells. During a germinal center response, these newly arising high-affinity auto-reactive B cells are subject to elimination. This has been demonstrated as their rescue by increased expression of endogenous or exogenous Bcl-2 [19]. It is not yet clear how their elimination occurs, although it can be postulated that it requires Ag engagement of the BCR in the absence of T cell help [20]. The antibodies are believed to mediate pathogenesis in lupus display somatic mutations and have undergone isotype class switching [21]. In an autoimmune-prone population, the BCR-mediated selection events that block the emergence of auto-reactive B cell at the germinal center stage are impaired. It is not known whether there can be a defect in germinal center selection without a defect in the selection of immature cells as the mechanisms of germinal center selection are poorly understood.