Lymphocyte populations and immune responses in CD5-deficient mice

Genomic organization and chromosomal localization of the human CD163 (M130) gene: a member of the scavenger receptor cysteine-rich superfamily

The human protein CD163 (M130) is a member of the scavenger receptor cysteine-rich (SRCR) superfamily, which is exclusively expressed by monocytes and macrophages. Here, we investigated the genomic organization and the chromosomal localization of the human CD163 gene. The CD163 gene is composed of 17 exons and 16 introns and spans over 35 kb. Each of its nine SRCR domains is encoded by a separate exon, which is similar to other members of the group B SRCR subfamily. Two cytoplasmic variants of CD163 arise from alternative splicing of intron 15, while a truncated and an extracellular variant results from alternative splicing of intron 5 or intron 7, respectively. Using fluorescence in situ hybridization we mapped this gene to the human chromosome 12p13. The transcription initiation sites of the CD163 gene were determined and the 5'-flanking region was sequenced. The nucleotide analysis revealed several putative binding sites for transcription factors, which have been shown to play an important role in myeloid specific gene expression. In addition, we identified a L1 element located 1.4 kb upstream of the major transcription initiation site.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

Visualization of CD4/CD8 T cell commitment

A system to innocuously visualize T cell lineage commitment is described. Using a "knock-in" approach, we have generated mice expressing a beta-galactosidase reporter in place of CD4; expression of beta-galactosidase in these animals appears to be an accurate and early indicator of CD4 gene transcription. We have exploited this knock-in line to trace CD4/CD8 lineage commitment in the thymus, avoiding important pitfalls of past experimental approaches. Our results argue in favor of a selective model of thymocyte commitment, demonstrating a fundamentally symmetrical process: engagement of either class of major histocompatibility complex (MHC) molecule by a differentiating CD4(+)CD8(+) cell can give rise to T cell antigen receptor (TCR)hi thymocytes of either lineage. Key findings include (a) direct demonstration of a substantial number of CD4-committed, receptor/coreceptor-mismatched cells in MHC class II- deficient mice, a critical prediction of the selective model; (b) highly efficient rescue of such "mismatched" intermediates by forced expression of CD8 in a TCR transgenic line, and an explanation of why previous experiments of this nature were less successful-a major past criticism of the selective model; (c) direct demonstration of an analogous, though smaller, population of CD8-committed mismatched intermediates in class I-deficient animals. Finally, we found no evidence of a CD4 default pathway.

CD5 expression is developmentally regulated by T cell receptor (TCR) signals and TCR avidity

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4(-)CD8(-) (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56(lck). At the CD4(+)CD8(+) (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR-major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4(+) or CD8(+) (single positive, SP) thymocytes is induced by engagement of the alpha/beta-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR-MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.

Interaction of the CD5 cytoplasmic domain with the Ca2+/calmodulin-dependent kinase IIdelta

CD5 is a type I transmembrane protein expressed on the surface of T cells and of B1 B cells. The analysis of CD5-deficient mice suggests that CD5 can down-regulate positive signals from the antigen receptors on T and B cells but the mechanism is not known at present. In contrast to the extracellular domain the 93 amino acid long cytoplasmic domain of CD5 is highly conserved between CD5 proteins of different mammalian species. Using the yeast two-hybrid system, we identified two proteins which specifically bind to the N-terminal part of the CD5 cytoplasmic sequence. These are the Ca2+/calmodulin-dependent kinase IIdelta and Tctex-1, a light chain component of the dynein motor complex. The interaction of CD5 with the Ca2+/calmodulin-dependent kinase IIdelta was reproduced in vitro using fusion proteins. The potential function of these proteins in CD5 internalization and negative signaling is discussed.

The Structure of Avian CD5 Implies a Conserved Function

The chicken CD5 cDNA was isolated by COS cell expression cloning utilizing a novel mAb 2-191. The cDNA contains a 1422-nucleotide open reading frame encoding a mature protein with 32% and 30% identity to mouse and human CD5 polypeptides, respectively. The molecule consists of a 330-amino acid extracellular region with three repeats of the scavenger receptor cysteine-rich domain, a 29-amino acid hydrophobic transmembrane domain, and a 93-amino acid cytoplasmic tail. The cytoplasmic region contains motifs that are highly conserved between species, including several potential phosphorylation sites. The chicken CD5 is a 64-kDa phosphorylated glycoprotein with a protein core of 57 kDa as determined by immunoprecipitation and SDS-PAGE analysis. αβ T cells express a homogeneously high level of CD5, whereas low or intermediate CD5 expression on γδ T cells depends on their tissue location. In contrast to human and mouse, CD5 is found at low levels on all chicken B cells. The high conservation of structural features, as well as signaling motifs, implies a conserved role for CD5 both in lymphocyte development and function.

CD5+ and CD5- B1-like lymphocytes in healthy guinea pig

Spleen, lymph node, and peripheral blood lymphocytes from healthy guinea pigs (gp) were examined for their ability to produce polyreactive autoantibodies to a battery of self-antigens and to cryptic determinants (phosphatidylcholine) on bromelain-treated mouse red blood cells (Br-MRBC). The mouse monoclonal antibody (Mab) 8BE6 anti-gp pan-T (CD5) marker was used for identification of CD5+ B1 cells by the plaque-forming assay (PFC), immunofluorescence, complement-mediated cytotoxicity, and immunocytochemistry. The detection of CD5+ cells by the 8BE6 Mab depended on the method used. They were better demonstrated by cytolysis and immunocytochemistry than by FACS analysis. By the latter method, the level of the CD5+ B cell subpopulation was associated neither with the age of the gp nor with the organ examined. Similarly wide ranges of PFC were detected in untreated or LPS-treated animals regardless of age and organ. The vast majority of the LPS-stimulated IgM antibody-secreting B lymphocytes reacting with the Br-MRBC, and those producing natural autoantibodies, did not bind the 8BE6 Mab.

CD6-ligand interactions: a paradigm for SRCR domain function?

The scavenger receptor cysteine-rich (SRCR) superfamily, which includes proteins expressed by leukocytes, can be subdivided into groups A and B. Group B contains the lymphocyte cell-surface receptor CD6. This article reviews recent progress in understanding the interaction between CD6 and its ligand, activated leukocyte cell adhesion molecule (ALCAM). Analysis of the CD6-ALCAM interaction may help to understand how other SRCR domains bind to their ligands.

CD5+ B cell-dependent regulation of the murine T-cell independent immune response against the human blood group A antigen

The CD5+B lymphocyte (B1a) population is known to be involved in most immune responses to microorganism TI antigens. Moreover, xid mice deficient for immune responses against TI-2 antigens are known to lack the B1a population, suggesting a role for B1a cells in TI-2 immune responses. We previously established that the oligosaccharide human blood group A antigen stimulated murine TI-2 immune responses. In this work, we show that the frequency of anti-A-secreting hybridomas was higher in mice with larger splenic B1a populations and that in vivo anti-CD5 treatment reduced anti-A immune response without affecting the response against TD RBC antigens. A similar effect was observed by in vitro anti-CD5 treatment of splenocytes. The in vivo anti-CD5 treatment also interfered with the immunization-dependent increase in splenocyte numbers. These results are in agreement with an important role for the B-cell CD5 receptor in the regulation of TI-2 immune responses possibly mediated by its interaction with the CD72 ligand.

Molecular cloning, mapping to human chromosome 1 q21-q23, and cell binding characteristics of Spalpha, a new member of the scavenger receptor cysteine-rich (SRCR) family of proteins

CD5 and CD6, two type I cell surface antigens predominantly expressed by T cells and a subset of B cells, have been shown to function as accessory molecules capable of modulating T cell activation. Here we report the cloning of a cDNA encoding Spalpha, a secreted protein that is highly homologous to CD5 and CD6. Spalpha has the same domain organization as the extracellular region of CD5 and CD6 and is composed of three SRCR (scavenger receptor cysteine rich) domains. Chromosomal mapping by fluorescence in situ hybridization and radiation hybrid panel analysis indicated that the gene encoding Spalpha is located on the long arm of human chromosome 1 at q21-q23 within contig WC1.17. RNA transcripts encoding Spalpha were found in human bone marrow, spleen, lymph node, thymus, and fetal liver but not in non-lymphoid tissues. Cell binding studies with an Spalpha immunoglobulin (Spalpha-mIg) fusion protein indicated that Spalpha is capable of binding to peripheral monocytes but not to T or B cells. Spalpha-mIg was also found to bind to the monocyte precursor cell lines K-562 and weakly to THP-1 but not to U937. Spalpha-mIg also bound to the B cell line Raji and weakly to the T cell line HUT-78. These findings indicate that Spalpha, a novel secreted protein produced in lymphoid tissues, may regulate monocyte activation, function, and/or survival.

CD2 Regulates the Positive Selection and Function of Antigen-Specific CD4−CD8+ T Cells

The CD2 glycoprotein has been implicated in both positive and negative regulation of T-cell mitogenesis. To study the involvement of CD2 in T-lymphocyte development and immune responses, we have analyzed two lines of CD2-null mice, each expressing a distinct class I major histocompatibility complex (MHC)-restricted T-cell receptor (TCR). In both situations, the absence of CD2 appeared to promote the positive selection of cells in a manner that is similar to that which occurs in the absence of CD5. Consistent with this, compound homozygotes that lacked both CD2 and CD5 showed evidence of enhanced positive selection even in the absence of a transgenic TCR. Despite the observed enhancement of positive selection, the lack of CD2 was associated with defects in proliferative responses and interferon-γ production when transgenic thymocytes and mature T lymphocytes were stimulated with the appropriate antigens. These findings raise the possibility that impaired sensitivity to selecting ligands in the thymus may provide a selective advantage that improves the efficiency of positive selection for certain TCRs. Furthermore, the results highlight the potential for a differential role for CD2 in thymocyte selection and T-cell immune responses.

The Ly-1.1 and Ly-1.2 epitopes of murine CD5 map to the membrane distal scavenger receptor cysteine-rich domain

CD5 is a member of a superfamily of proteins which contain one or more extracellular domains homologous to the type I macrophage Scavenger Receptor cysteine-rich (SRCR) domain. The extracellular region of CD5 is composed of three SRCR domains (D1, D2, D3). Murine CD5 (mCD5) is polymorphic (Ly-1.1 and Ly-1.2 alleles), however, the only murine CD5 gene characterized to date encodes the Ly-1.2 allele (mCD5.2). Likewise, the domain specificity of many of the available anti-mCD5 mAb recognizing either Ly-1.1 or Ly-1.2 or both has not been examined. Herein we describe the isolation and characterization of cDNA encoding the Ly1.1 allele (mCD5.1) and map the location and molecular nature of the mCD5 allelic variation. We also determined which SRCR domain of mCD5 is recognized by a panel of anti-mCD5 mAb. The mCD5.1 protein differs from mCD5.2 in only three amino acids, all of which map to the most amino terminal SRCR domain (D1) of mCD5. An additional seven silent substitutions were observed in the nucleotide sequence encoding mCD5 D1, D2 and transmembrane domains. Immunoglobulin (Ig) fusion proteins consisting of various combinations of mCD5.1 or mCD5.2 SRCR domains were produced and used to determine that allele specific mAb bound to D1, confirming sequence data. MAb against monomorphic determinants on mCD5 bound to each mCD5D11g.

B cell development in mice

The development and establishment of the B Cell Repertoire is the net result of both genetic and environmental forces. The primary event at the genetic level is Ig gene rearrangement resulting in numerous possible combination of genes which can be further modified by somatic events such as N segment addition and somatic mutation. Environmental forces in the form of self and exogenous Ags also shape the repertoire by positively or negatively selecting B cells according to the specificity of their Ig receptors. These are dynamic processes beginning with the earliest expression of immunoglobulins in fetal life and continuing throughout life. In this review we discuss the genetic and selective mechanisms responsible for differences in the early immune system compared to that of the adult.

CD5-mediated negative regulation of antigen receptor-induced growth signals in B-1 B cells

A subset of B lymphocytes present primarily in the peritoneal and pleural cavities is defined by the expression of CD5 and is elevated in autoimmune diseases. Upon signaling through membrane immunoglobulin M (mIgM), splenic B lymphocytes (B-2) proliferate, whereas peritoneal B cells (B-1) undergo apoptosis. However, in CD5-deficient mice, B-1 cells responded to mIgM crosslinking by developing a resistance to apoptosis and entering the cell cycle. In wild-type B-1 cells, prevention of association between CD5 and mIgM rescued their growth response to mIgM crosslinking. Thus the B cell receptor-mediated signaling is negatively regulated by CD5 in normal B-1 cells.

PCR-based gene targeting of the inducible nitric oxide synthase (NOS2) locus in murine ES cells, a new and more cost-effective approach

Gene targeting by double homologous recombination in murine embryonic stem (ES) cells is a powerful tool used to study the cellular consequences of specific genetic mutations. A typical targeting construct consists of a neomycin phosphotransferase (neo) gene flanked by genomic DNA fragments that are homologous to sequences in the target chromosomal locus. Homologous DNA fragments are typically cloned from a murine genomic DNA library. Here we describe an alternative approach whereby the inducible nitric oxide synthase (NOS2) gene locus is partially mapped and homologous DNA sequences obtained using a long-range PCR method. A 7 kb NOS2 amplicon is used to construct a targeting vector where the neo gene is flanked by PCR-derived homologous DNA sequences. The vector also includes a thymidine kinase (tk) negative-selectable marker gene. Following transfection into ES cells, the PCR-based targeting vector undergoes efficient homologous recombination into the NOS2 locus. Thus, PCR-based gene targeting can be a valuable alternative to the conventional cloning approach. It expedites the acquisition of homologous genomic DNA sequences and simplifies the construction of targeting plasmids by making use of defined cloning sites. This approach should result in substantial time and cost savings for appropriate homologous recombination projects.

Lymphocyte populations in atherosclerotic lesions of apoE -/- and LDL receptor -/- mice. Decreasing density with disease progression

Lymphocytes are prominent components of human atherosclerotic lesions, but their presence in murine models of disease has not been confirmed. Lymphocyte subpopulations have been identified in apoE -/- and LDL receptor -/- mice fed a cholesterol-enriched diet for up to 3 months. ApoE -/- mice had higher serum cholesterol concentrations than did LDL receptor -/- mice during most of the feeding period, primarily due to large increases in VLDL concentrations. Total area of atherosclerotic lesions was greater at all times in apoE -/- than LDL receptor -/- mice (lesion area after 3 months on cholesterol-enriched diet: apoE -/-, 993 +/- 193 and LDL receptor -/-, 560 +/- 131 microns2 x 10(3), mean +/- SEM, n = 6 in each group). Lesions in apoE -/- mice contained larger macrophage-rich necrotic cores and more calcification than did those in LDL receptor -/- mice. Immunocytochemical analyses of tissue sections of ascending aortas performed with monoclonal antibodies to T and B lymphocytes and macrophages revealed that T lymphocytes immunoreactive for Thy 1.2, CD5, CD4, and CD8 were observed in lesions from both strains, but no B lymphocytes were detected. The density of Thy 1.2+ T lymphocytes in lesions was greatest at 1 month (apoE -/-, 98 +/- 23 and LDL receptor -/-, 201 +/- 40 lymphocytes/mm2, n = 6 in each group), decreasing in apoE -/- mice to 12 +/- 3 and in LDL receptor -/- mice to 51 +/- 20 lymphocytes/mm2 at 3 months. The presence of T lymphocytes in murine atherosclerotic lesions makes these animals potentially useful for studying the involvement of the immune system in atherogenesis.

Isotype and antibody specificity of spontaneously formed immunoglobulins in pig fetuses and germ-free piglets: production by CD5- B cells

Pig fetuses, colostrum-deprived newborns and germ-free (GF) piglets, animals in which B-cell development is not influenced by maternal regulatory factors, were employed to study the occurrence and specificity of natural antibodies (NAb). Serum immunoglobulins of all isotypes were found in 44-day-old fetuses (the gestation period in pigs lasts 114 days) and their level, with predominating IgM, was increased during fetal ontogeny. In sera of fetuses at the end of embryonic life as well as of newborns and older GF piglets, antibody activity against autoantigens (thyroglobulin, hormones, ssDNA), phylogenetically conserved proteins (myosin), haptens (trinitrophenyl; TNP) and bacterial components (Escherichia coli O86, tetanic anatoxin) was detected by enzyme-linked immunosorbent assay. The antigen-biding activity of IgM NAb increased after isolation of the serum immunoglobulins on a Staphylococcus Protein A (SPA)-Sepharose column. IgM reactivity similar to that detected in serum was found in supernatants from polyclonally stimulated cultures of spleen of 8- and 12-day-old GF piglets. Pig fetal liver IgM+ B cells, which were able to produce IgM after polyclonal stimulation, did not express the CD5 molecule. Our results indicate that pig preimmune repertoire is comparable to that described in humans and mice, although in contrast to these species pig B-1 cells do not express CD5.

Polyclonal immunoglobulin response of thymic, hepatic and splenic lymphocytes from fetal, germ-free and conventionally reared pigs to different B-cell activators

Immunoglobulin (Ig) response to different polyclonal B-cell activators was measured by ELISA in cell culture media of thymocytes, splenocytes and liver cells isolated from pig fetuses, 8-d-old germ-free piglets and conventionally reared pigs. Both in fetal and in postnatal life polyclonally stimulated lymphocytes were found to produce predominantly the IgM isotype; the first IgM formation was detected in 50-d-old fetal liver (gestation in pigs lasts 114 d). Surprisingly, 73-d-old fetal thymic cells were shown to be induced to Ig synthesis and secretion. In contrast to splenocytes of the same age, which secreted exclusively IgM, fetal thymocytes produced IgM, IgG and IgA. Polyclonally stimulated splenic cells as compared with thymic cells started to produce IgA later in fetal ontogeny, whereas the IgG response was not detectable in splenic cell culture media during the whole embryonal development and appeared only after birth. The earliest and the highest Ig stimulation was found after cultivation of lymphocytes with Nocardia delipidated cell mitogen. Interestingly, the moderate stimulatory effect of 65-kDa heat shock protein (Hsp-65) in polyclonal IgM response of fetal splenocytes was observed. We showed that thymic B lymphocytes represent probably the first maturing B cell population detectable in fetal life, which is able to differentiate after polyclonal stimulation into IgM as well as IgA and IgG producing cells.

A role for CD5 in TCR-mediated signal transduction and thymocyte selection

CD5 is a transmembrane protein that is expressed on the surface of T cells and a subset of B cells. The absence of CD5 rendered thymocytes hyperresponsive to stimulation through the T cell antigen receptor (TCR) in vitro. Selection of T cells expressing three distinct transgenic TCRs was also abnormal in CD5-deficient mice. These observations indicate that CD5 can influence the fate of developing thymocytes by acting as a negative regulator of TCR-mediated signal transduction.

Phenotypic changes that TCR V gamma 3+ fetal thymocytes undergo during their maturation into dendritic epidermal T cells

Murine Thy-1+, TCR V gamma 3/V delta 1+ dendritic epidermal T cells (DETC) express CD2 antigens, but differ from most other T-cell subsets in their absence of CD4, CD5, and CD8 antigens. To determine whether negativity for those antigens is an intrinsic feature of a given T-cell population or whether such triple-negative T cells go through a maturational stage during which they express these antigens, we determined the phenotype of TCR V gamma 3+ fetal thymocytes, which are the precursor cells of DETC. We found that TCR V gamma 3+ fetal thymocytes at day 17 of gestation are CD2+, CD5+, mostly CD8+, and partly CD4+. The expression of CD5 is highest on early TCR V gamma 3+ thymocytes; these cells express intermediate levels of CD5 when they leave the thymus and lose CD5 expression until or shortly after arrival in the epidermis. A similar loss of CD5 expression by TCR V gamma 3+ cells was observed in vitro under various culture conditions. To determine whether expression of CD5 is important for the maturation of DETC, we searched for these cells in the epidermis of CD5-deficient mice. There was no alteration in the number of Thy-1+/TCR V gamma 3+ dendritic cells in the epidermis of CD5-/- mice. Even though the latter finding speaks against a pivotal role of CD5 during the maturation of DETC, the described cell system may serve as a useful tool in further experiments aimed to clarify the function of the CD5 glycoprotein as well as the mechanism(s) regulating its expression.

B-cell activation by crosslinking of surface IgM or ligation of CD40 involves alternative signal pathways and results in different B-cell phenotypes

Treatment of small resting B cells with soluble F(ab')2 fragments of anti-IgM, an analogue of T-independent type 2 antigens, induced activation characterized by proliferation and the expression of surface CD5. In contrast, B cells induced to proliferate in response to thymus-dependent inductive signals provided by either fixed activated T-helper 2 cells or soluble CD40 ligand-CD8 (CD40L) recombinant protein displayed elevated levels of CD23 (Fc epsilon II receptor) and no surface CD5. Treatment with anti-IgM and CD40L induced higher levels of proliferation and generated a single population of B cells coexpressing minimal amounts of CD5 and only a slight elevation of CD23. Anti-IgM- but not CD40L-mediated activation was highly sensitive to inhibition by cyclosporin A and FK520. Sp-cAMPS, an analogue of cAMP, augmented CD40L and suppressed surface IgM-mediated activation. Taken together these results are interpreted to mean that there is a single population of small resting B cells that can respond to either T-independent type 2 (surface IgM)- or T-dependent (CD40)-mediated activation. In response to different intracellular signals these cells are induced to enter alternative differentiation pathways.

Expression of cell interaction molecules by immature rat thymocytes during passage through the CD4+8+ compartment: developmental regulation and induction by T cell receptor engagement of CD2, CD5, CD28, CD11a, CD44 and CD53

Rat thymocytes of the T cell receptorlow (TcRlow) CD4+8+ subset which is the target of repertoire selection are heterogeneous with respect to expression of the cell interaction (CI) molecules CD2, CD5, CD11a/CD18 (LFA-1), CD28 and CD44. We show that this heterogeneity is due to the developmental regulation of these CI molecules during passage through the CD4+8+ compartment, and to up-regulation by TcR engagement. Thus, cohorts of CD4+8+ cells differentiating synchronously in vitro from their direct precursors, the immature CD4-8+ cells, were homogeneous with regard to CI molecule expression. Upon entry into the CD4+8+ compartment, they expressed relatively high levels of CD2 and CD44, and moderate levels of CD5, CD28 and CD11a. CD2, CD28 and CD44 were slightly down-regulated during the following 2 days, whereas CD5 slightly increased and CD11a remained constant. TcR stimulation using immobilized monoclonal antibodies resulted in rapid and dramatic up-regulation of CD2, CD5 and CD28 and, to a lesser extent, of CD11a and CD44. Finally CD53, a triggering structure absent from unstimulated CD4+8+ thymocytes was also rapidly induced by TcR stimulation. Inclusion of interleukin (IL)-2, IL-4, or IL-7 in this in vitro differentiation system did not affect the levels of CI molecules studied. Since the high levels of CI molecules induced by TcR-stimulation correspond to those found in vivo on TcRintermediate thymocytes known to be undergoing repertoire selection, these results suggest that upregulation of CI molecules by TcR engagement provides a mechanism by which thymocytes that have entered the selection process gain preferential access to further interactions with stromal and lymphoid cells in the thymus.