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IntroductionPlasma cells (PCs) are terminally differentiated immunoglobulin (Ig)-secreting cells (ISCs) responsible for humoral immunity. [1][2][3] ISCs can arise from several differentiation pathways. During the extrafollicular reaction, some antigen (Ag)-specific naive B cells undergo rapid clonal expansion and differentiate into ISCs. 1,4,5 Many extrafollicular ISCs are short-lived and secrete either IgM or downstream isotypes. 6,7 The V H genes of extrafollicular ISCs typically remain in their germline configuration, 4,8 indicating they have not undergone significant somatic mutation. Naive B cells can also give rise to ISCs by way of the germinal center (GC) reaction. It is within GC that antigen-specific naive B cells undergo proliferation, somatic mutation of Ig V region genes, Ig isotype switching, and affinity maturation. 1,4,5 Antigen-selected GC B cells can differentiate into ISCs, 5,9,10 most of which migrate to the bone marrow (BM) and become long-lived PCs, whereas a small proportion remains in the spleen. 2,3,7,9,11,12 At the same time, some GC B cells develop into memory B cells, 4,5,10 which can subsequently yield ISCs directly after repeated exposure to an immunizing antigen. 13,14 Thus, the GC reaction serves to expand and diversify the B-cell repertoire, producing high-affinity variants for the long-term maintenance of protective immunity.Most studies examining PCs have been performed using spleens and BM from immunized rodents. However, for a complete understanding of human PC differentiation, and, more important, human diseases characterized by the expansion or transformation of ISCs, it is necessary to examine the human counterpart of these cells directly. Human ISCs can be identified by an up-regulation in expression of CD38 and a concomitant down-regulation of CD20. 15 Based on this phenotype, significant numbers have been detected in BM and peripheral blood (PB) 15,16 and in mucosa-associated lymphoid tissue such as tonsils and gut, [17][18][19] where they usually constitute a very small proportion of mononuclear cells (MNCs; less than 0.1%-1.0%). 15,17,20,21 Although ISCs in human tonsils, PB, and BM have been extensively characterized, 16,17,[20][21][22][23][24][25] ISCs in human spleen have not been reported. Given the identification of a population of long-lived murine splenic PCs, we hypothesized that an analogous population would exist in humans and that this population could contribute to the maintenance of long-term humoral immunity. In this paper we report the identification of a population of human splenic CD38 ϩϩ CD20 Ϯ cells that exhibited many characteristics of BM PCs, and we propose that these cells in human spleen represent an important population of ISCs. Identifying human splenic ISCs will contribute to our understanding of the role played by these cells in normal humoral immunity and of human diseases characterized by B-cell hyperactivity or deficiency. Materials and methods ReagentsThe following monoclonal antibodies (mAbs) were used in this study: fluorescein isothi...

The role of glutamine and alanine transport in the recycling of neurotransmitter glutamate was investigated in Guinea pig brain cortical tissue slices and prisms, and in cultured neuroblastoma and astrocyte cell lines. The ability of exogenous (2 mM) glutamine to displace 13 C label sup-13 C]glucose was investigated using NMR spectroscopy.Glutamine transport was inhibited in slices under quiescent or depolarising conditions using histidine, which shares most transport routes with glutamine, or 2-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of the neuronal system A. Glutamine mainly entered a large, slow turnover pool, probably located in neurons, which did not interact with the glutamate/glutamine neurotransmitter cycle. Abbreviations used: ASCT-2, system ASC amino acid transporter; ATA1 and ATA2, system A amino acid transporters; EAAT, excitatory amino acid transporter; HBSS, HEPES-buffered salt solution; MeAIB, 2-(methylamino)isobutyric acid; RPMI, Roswell Park Memorial Institute Medium 1640.

X-linked lymphoproliferative disease (XLP) is an often-fatal immunodeficiency characterized by hypogammaglobulinemia, fulminant infectious mononucleosis, and/or lymphoma. The genetic lesion in XLP, SH2D1A, encodes the adaptor protein SAP (signaling lymphocytic activation molecule-associated [SLAM-associated] protein); however, the mechanism(s) by which mutations in SH2D1A causes hypogammaglobulinemia is unknown. Our analysis of 14 XLP patients revealed normal B cell development but a marked reduction in the number of memory B cells. The few memory cells detected were IgM(+), revealing deficient isotype switching in vivo. However, XLP B cells underwent proliferation and differentiation in vitro as efficiently as control B cells, which indicates that the block in differentiation in vivo is B cell extrinsic. This possibility is supported by the finding that XLP CD4(+) T cells did not efficiently differentiate into IL-10(+) effector cells or provide optimal B cell help in vitro. Importantly, the B cell help provided by SAP-deficient CD4(+) T cells was improved by provision of exogenous IL-10 or ectopic expression of SAP, which resulted in increased IL-10 production by T cells. XLP CD4(+) T cells also failed to efficiently upregulate expression of inducible costimulator (ICOS), a potent inducer of IL-10 production by CD4(+) T cells. Thus, insufficient IL-10 production may contribute to hypogammaglobulinemia in XLP. This finding suggests new strategies for treating this immunodeficiency.