Lymphocyte populations and immune responses in CD5-deficient mice

Internal regulation between constitutively expressed T cell co-inhibitory receptors BTLA and CD5 and tolerance in recent thymic emigrants

Immunologic self-tolerance involves signals from co-inhibitory receptors. Several T cell co-inhibitors, including PD-1, are expressed upon activation, whereas CD5 and BTLA are expressed constitutively. The relationship between constitutively expressed co-inhibitors and when they are needed is unknown. Deletion of Btla demonstrated BTLA regulates CD5 expression. Loss of BTLA signals, but not signalling by its ligand, HVEM, leads to increased CD5 expression. Higher CD5 expression set during thymic selection is associated with increased self-recognition, suggesting that BTLA might be needed early to establish self-tolerance. We found that BTLA and PD-1 were needed post-thymic selection in recent thymic emigrants (RTE). RTE lacking BTLA caused a CD4 T cell and MHC class II dependent multi-organ autoimmune disease. Together, our findings identify a negative regulatory pathway between two constitutively expressed co-inhibitors, calibrating their expression. Expression of constitutive and induced co-inhibitory receptors is needed early to establish tolerance in the periphery for RTE.

THEMIS increases TCR signaling in CD4+CD8+ thymocytes by inhibiting the activity of the tyrosine phosphatase SHP1

The T cell lineage-restricted protein THEMIS plays a critical role in T cell development at the positive selection stage. In the SHP1 activation model, THEMIS is proposed to enhance the activity of the tyrosine phosphatase SHP1 (encoded by Ptpn6), thereby dampening T cell antigen receptor (TCR) signaling and preventing the inappropriate negative selection of CD4+CD8+ thymocytes by positively selecting ligands. In contrast, in the SHP1 inhibition model, THEMIS is proposed to suppress SHP1 activity, rendering CD4+CD8+ thymocytes more sensitive to TCR signaling initiated by low-affinity ligands to promote positive selection. We sought to resolve the controversy regarding the molecular function of THEMIS. We found that the defect in positive selection in Themis-/- thymocytes was ameliorated by pharmacologic inhibition of SHP1 or by deletion of Ptpn6 and was exacerbated by SHP1 overexpression. Moreover, overexpression of SHP1 phenocopied the Themis-/- developmental defect, whereas deletion of Ptpn6, Ptpn11 (encoding SHP2), or both did not result in a phenotype resembling that of Themis deficiency. Last, we found that thymocyte negative selection was not enhanced but was instead impaired in the absence of THEMIS. Together, these results provide evidence favoring the SHP1 inhibition model, supporting a mechanism whereby THEMIS functions to enhance the sensitivity of CD4+CD8+ thymocytes to TCR signaling, enabling positive selection by low-affinity, self-ligand-TCR interactions.

Experimental and genetic evidence for the impact of CD5 and CD6 expression and variation in inflammatory bowel disease

Crohn’s disease (CD) and ulcerative colitis (UC) are inflammatory bowel diseases (IBD) resulting from the interaction of multiple environmental, genetic and immunological factors. CD5 and CD6 are paralogs encoding lymphocyte co-receptors involved in fine-tuning intracellular signals delivered upon antigen-specific recognition, microbial pattern recognition and cell adhesion. While CD5 and CD6 expression and variation is known to influence some immune-mediated inflammatory disorders, their role in IBD remains unclear. To this end, Cd5- and Cd6-deficient mice were subjected to dextran sulfate sodium (DSS)-induced colitis, the most widely used experimental animal model of IBD. The two mouse lines showed opposite results regarding body weight loss and disease activity index (DAI) changes following DSS-induced colitis, thus supporting Cd5 and Cd6 expression involvement in the pathophysiology of this experimental IBD model. Furthermore, DNA samples from IBD patients of the ENEIDA registry were used to test association of CD5 (rs2241002 and rs2229177) and CD6 (rs17824933, rs11230563, and rs12360861) single nucleotide polymorphisms with susceptibility and clinical parameters of CD (n=1352) and UC (n=1013). Generalized linear regression analyses showed association of CD5 variation with CD ileal location (rs2241002^CC) and requirement of biological therapies (rs2241002^C-rs2229177^T haplotype), and with poor UC prognosis (rs2241002^T-rs2229177^T haplotype). Regarding CD6, association was observed with CD ileal location (rs17824933^G) and poor prognosis (rs12360861^G), and with left-sided or extensive UC, and absence of ankylosing spondylitis in IBD (rs17824933^G). The present experimental and genetic evidence support a role for CD5 and CD6 expression and variation in IBD’s clinical manifestations and therapeutic requirements, providing insight into its pathophysiology and broadening the relevance of both immunomodulatory receptors in immune-mediated disorders.

CD5 Controls Gut Immunity by Shaping the Cytokine Profile of Intestinal T Cells

CD5 is constitutively expressed on all T cells and is a negative regulator of lymphocyte function. However, the full extent of CD5 function in immunity remains unclear. CD5 deficiency impacts thymic selection and extra-thymic regulatory T cell generation, yet CD5 knockout was reported to cause no immune pathology. Here we show that CD5 is a key modulator of gut immunity. We generated mice with inducible CD5 knockdown (KD) in the autoimmune-prone nonobese diabetic (NOD) background. CD5 deficiency caused T cell-dependent wasting disease driven by chronic gut immune dysregulation. CD5 inhibition also exacerbated acute experimental colitis. Mechanistically, loss of CD5 increased phospho-Stat3 levels, leading to elevated IL-17A secretion. Our data reveal a new facet of CD5 function in shaping the T cell cytokine profile.

The rational development of CD5-targeting biepitopic CARs with fully human heavy-chain-only antigen recognition domains

T cell malignancies are a group of hematologic cancers with high recurrence and mortality rates. CD5 is highly expressed in ∼85% of T cell malignancies, although normal expression of CD5 is restricted to thymocytes, T cells, and B1 cells. However, CD5 expression on chimeric antigen receptor (CAR)-T cells leads to CAR-T cell fratricide. Once this limitation is overcome, CD5-targeting CAR-T therapy could be an attractive strategy to treat T cell malignancies. Here, we report the selection of novel CD5-targeting fully human heavy-chain variable (FHV_H) domains for the development of a biepitopic CAR, termed FHV_H3/V_H1, containing FHV_H1 and FHV_H3, which were validated to bind different epitopes of the CD5 antigen. To prevent fratricide in CD5 CAR-T cells, we optimized the manufacturing procedures of a CRISPR-Cas9-based CD5 knockout (CD5KO) and lentiviral transduction of anti-CD5 CAR. In vitro and in vivo functional comparisons demonstrated that biepitopic CD5KO FHV_H3/V_H1 CAR-T cells exhibited enhanced and longer lasting efficacy; produced moderate levels of cytokine secretion; showed similar specificity profiles as either FHV_H1, FHV_H3, or the clinically tested H65; and is therefore suitable for further development.

CD5 dynamically calibrates basal NF-κB signaling in T cells during thymic development and peripheral activation

Immature T cells undergo a process of positive selection in the thymus when their new T cell receptor (TCR) engages and signals in response to self-peptides. As the T cell matures, a slew of negative regulatory molecules, including the inhibitory surface glycoprotein CD5, are up-regulated in proportion to the strength of the self-peptide signal. Together these regulators dampen TCR-proximal signaling and help avoid any subsequent peripheral activation of T cells by self-peptides. Paradoxically, antigen-specific T cells initially expressing more CD5 (CD5^hi) have been found to better persist as effector/memory cells after a peripheral challenge. The molecular mechanisms underlying such a duality in CD5 function is not clear. We found that CD5 alters the basal activity of the NF-κB signaling in resting peripheral T cells. When CD5 was conditionally ablated, T cells were unable to maintain higher expression of the cytoplasmic NF-κB inhibitor IκBα. Consistent with this, resting CD5^hi T cells expressed more of the NF-κB p65 protein than CD5^lo cells, without significant increases in transcript levels, in the absence of TCR signals. This posttranslationally stabilized cellular NF-κB depot potentially confers a survival advantage to CD5^hi T cells over CD5^lo ones. Taken together, these data suggest a two-step model whereby the strength of self-peptide-induced TCR signal lead to the up-regulation of CD5, which subsequently maintains a proportional reserve of NF-κB in peripheral T cells poised for responding to agonistic antigen-driven T cell activation.

TLR induces reorganization of the IgM-BCR complex regulating murine B-1 cell responses to infections

In mice, neonatally-developing, self-reactive B-1 cells generate steady levels of natural antibodies throughout life. B-1 cells can, however, also rapidly respond to infections with increased local antibody production. The mechanisms regulating these two seemingly very distinct functions are poorly understood, but have been linked to expression of CD5, an inhibitor of BCR-signaling. Here we demonstrate that TLR-mediated activation of CD5+ B-1 cells induced the rapid reorganization of the IgM-BCR complex, leading to the eventual loss of CD5 expression, and a concomitant increase in BCR-downstream signaling, both in vitro and in vivo after infections of mice with influenza virus and Salmonella typhimurium. Both, initial CD5 expression and TLR-mediated stimulation, were required for the differentiation of B-1 cells to IgM-producing plasmablasts after infections. Thus, TLR-mediated signals support participation of B-1 cells in immune defense via BCR-complex reorganization.

The multiple faces of CD5

Since its discovery, over 30 years ago, CD5 has been used as a marker to identify T cells, B1-a cells, and B cell chronic lymphocytic leukemia cells. Throughout the years, many studies have described the functional relevance of CD5 as a modulator of T and B cell receptor signaling. However, it has not been until recent years that CD5 has emerged as a functional receptor in other areas of the immune system. Here, we review some of the most important aspects of CD5 as a modulator of TCR and BCR signaling, cell survival receptor both in T and B cells during health and disease, as well as the newly discovered roles of this receptor in thymocyte selection, T cell effector differentiation, and immune tolerance. CD5 was found to promote T cell survival by protecting autoreactive T cell from activation-induced cell death, to promote de novo induction of regulatory T cells in the periphery, to modulate Th17 and Th2 differentiation, and to modulate immune responses by modulating dendritic cell functions. CD5 is overexpressed in Tregs and Bregs, which are fundamental to maintain immune homeostasis. The newly established roles of CD5 in modulating different aspects of immune responses identify this receptor as an immune checkpoint modulator, and therefore it could be used as a target for immune intervention in different pathologies such as cancer, autoimmune diseases or infections.

CD6, a Rheostat-Type Signalosome That Tunes T Cell Activation

Following T cell receptor triggering, T cell activation is initiated and amplified by the assembly at the TCR/CD3 macrocomplex of a multitude of stimulatory enzymes that activate several signaling cascades. The potency of signaling is, however, modulated by various inhibitory components already at the onset of activation, long before co-inhibitory immune checkpoints are expressed to help terminating the response. CD5 and CD6 are surface glycoproteins of T cells that have determinant roles in thymocyte development, T cell activation and immune responses. They belong to the superfamily of scavenger receptor cysteine-rich (SRCR) glycoproteins but whereas the inhibitory role of CD5 has been established for long, there is still controversy on whether CD6 may have similar or antagonistic functions on T cell signaling. Analysis of the structure and molecular associations of CD5 and CD6 indicates that these molecules assemble at the cytoplasmic tail a considerable number of signaling effectors that can putatively transduce diverse types of intracellular signals. Biochemical studies have concluded that both receptors can antagonize the flow of TCR-mediated signaling; however, the impact that CD5 and CD6 have on T cell development and T cell-mediated immune responses may be different. Here we analyze the signaling function of CD6, the common and also the different properties it exhibits comparing with CD5, and interpret the functional effects displayed by CD6 in recent animal models.

Downregulation of CD5 and dysregulated CD8+ T-cell activation

CD5 is a cell surface molecule that is expressed on most circulating T cells and a small population of B cells, and is involved in modulation of antigen-specific receptor-mediated activation. Downregulation of CD5 on CD8⁺ T cells is a poorly understood but increasingly recognized phenomenon that may be associated with dysregulated T-cell activation. An increased subpopulation of activated CD8⁺ T cells with downregulation of CD5 has recently been described in patients with Epstein-Barr virus-associated hemophagocytic lymphohistiocytosis (HLH) and familial HLH caused by perforin deficiency and Munc 13-4 deficiency. These cells were detectable only in the acute phase of HLH, in which patients exhibited hypercytokinemia, and declined progressively after successful treatment in parallel with improvement of systemic inflammation. It is unknown whether CD8⁺ T cells from HLH with other causes have similar profiles. Assessment of CD5 expression on T cells has the potential to assist in the understanding of the diagnosis and pathogenesis of human inflammatory diseases such as HLH.

Animal models for studying tumor microenvironment (TME) and resistance to lymphocytic infiltration

In cancer immunotherapy, cytotoxic T or NK cells need to engage cancer cells to initiate the killing. However, in clinical studies and in mouse models, some solid tumors are found with no lymphocytes. It is likely that these tumors will be resistant to all sorts of immunotherapies. Thus, restoring lymphocytic infiltration will be vital to the success of immunotherapies on solid tumors. In order to understand the complex interaction between cancer cells and stromal cells, we propose to establish animal models for studying the tumor microenvironment and to develop and test therapies to restore lymphocytic infiltration of tumors Without lymphocytes infiltrating tumors, all immunotherapies on solid tumors become ineffective.

Tuning T Cell Signaling Sensitivity Alters the Behavior of CD4+ T Cells during an Immune Response

Intricate processes in the thymus and periphery help curb the development and activation of autoreactive T cells. The subtle signals that govern these processes are an area of great interest, but tuning TCR sensitivity for the purpose of affecting T cell behavior remains technically challenging. Previously, our laboratory described the derivation of two TCR-transgenic CD4 T cell mouse lines, LLO56 and LLO118, which recognize the same cognate Listeria epitope with the same affinity. Despite the similarity of the two TCRs, LLO56 cells respond poorly in a primary infection whereas LLO118 cells respond robustly. Phenotypic examination of both lines revealed a substantial difference in their surface of expression of CD5, which serves as a dependable readout of the self-reactivity of a cell. We hypothesized that the increased interaction with self by the CD5-high LLO56 was mediated through TCR signaling, and was involved in the characteristic weak primary response of LLO56 to infection. To explore this issue, we generated an inducible knock-in mouse expressing the self-sensitizing voltage-gated sodium channel Scn5a. Overexpression of Scn5a in peripheral T cells via the CD4-Cre promoter resulted in increased TCR-proximal signaling. Further, Scn5a-expressing LLO118 cells, after transfer into BL6 recipient mice, displayed an impaired response during infection relative to wild-type LLO118 cells. In this way, we were able to demonstrate that tuning of TCR sensitivity to self can be used to alter in vivo immune responses. Overall, these studies highlight the critical relationship between TCR-self-pMHC interaction and an immune response to infection.

SK, Montero E, Pai HV, Palanivelu DV, Melarkode R, Nair P. T cell activation and differentiation is modulated by a CD6 domain 1 antibody Itolizumab

CD6 is associated with T-cell modulation and is implicated in several autoimmune diseases. We previously demonstrated that Itolizumab, a CD6 domain 1 (CD6D1) specific humanized monoclonal antibody, inhibited the proliferation and cytokine production by T lymphocytes stimulated with anti-CD3 antibody or when co-stimulated with ALCAM. Aberrant IL-17 producing CD4+ helper T-cells (Th17) have been identified as pivotal for the pathogenesis of certain inflammatory autoimmune disorders, including psoriasis. Itolizumab has demonstrated efficacy in human diseases known to have an IL-17 driven pathogenesis. Here, in in vitro experiments we show that by day 3 of human PBMC activation using anti-CD3 and anti-CD28 co-stimulation in a Th17 polarizing milieu, 15-35% of CD4+ T-cells overexpress CD6 and there is an establishment of differentiated Th17 cells. Addition of Itolizumab reduces the activation and differentiation of T cells to Th17 cells and decreases production of IL-17. These effects are associated with the reduction of key transcription factors pSTAT3 and RORγT. Further, transcription analysis studies in these conditions indicate that Itolizumab suppressed T cell activation by primarily reducing cell cycle, DNA transcription and translation associated genes. To understand the mechanism of this inhibition, we evaluated the effect of this anti-human CD6D1 mAb on ALCAM-CD6 as well as TCR-mediated T cell activation. We show that Itolizumab but not its F(ab')2 fragment directly inhibits CD6 receptor hyper-phosphorylation and leads to subsequent decrease in associated ZAP70 kinase and docking protein SLP76. Since Itolizumab binds to CD6 expressed only on human and chimpanzee, we developed an antibody binding specifically to mouse CD6D1. This antibody successfully ameliorated the incidence of experimental autoimmune encephalitis in the mice model. These results position CD6 as a key molecule in sustaining the activation and differentiation of T cells and an important target for modulating autoimmune diseases.

Immunomodulatory Functions of BTLA and HVEM Govern Induction of Extrathymic Regulatory T Cells and Tolerance by Dendritic Cells

Dendritic cells (DCs) initiate immunity and also antigen-specific tolerance mediated by extrathymic regulatory T (Treg) cells, yet it remains unclear how DCs regulate induction of such tolerance. Here, we report that efficient induction of Treg cells was instructed by BTLA⁺DEC205⁺CD8⁺CD11c⁺ DCs and the immunomodulatory functions of BTLA. In contrast, T cell activation in steady state by total CD11c⁺ DCs that include a majority of DCs that do not express BTLA did not induce Treg cells and had no lasting impact on subsequent immune responses. Engagement of HVEM, a receptor of BTLA, promoted Foxp3 expression in T cells through upregulation of CD5. In contrast, T cells activated in the absence of BTLA and HVEM-mediated functions remained CD5^lo and therefore failed to resist the inhibition of Foxp3 expression in response to effector cell-differentiating cytokines. Thus, DCs require BTLA and CD5-dependent mechanisms to actively adjust tolerizing T cell responses under steady-state conditions.

Functional Heterogeneity in CD4(+) T Cell Responses Against a Bacterial Pathogen

To investigate how CD4⁺ T cells function against a bacterial pathogen, we generated a Listeria monocytogenes-specific CD4⁺ T cell model. In this system, two TCRtg mouse lines, LLO56 and LLO118, recognize the same immunodominant epitope (LLO_190-205) of L. monocytogenes and have identical in vitro responses. However, in vivo LLO56 and LLO118 display vastly different responses during both primary and secondary infection. LLO118 dominates in the primary response and in providing CD8 T cell help. LLO56 predominates in the secondary response. We have also shown that both specific [T cell receptor (TCR)-mediated] and non-specific stimuli (bypassing the TCR) elicit distinct responses from the two transgenics, leading us to conclude that the strength of self-pMHC signaling during development tightly dictates the cell’s future response in the periphery. Herein, we review our findings in this transfer system, focusing on the contribution of the immunomodulatory molecule CD5 and the importance of self-interaction in peripheral maintenance of the cell. We also discuss the manner in which individual TCR affinities to foreign and self-pMHC contribute to the outcome of an immune response; our assertion is that there exists a spectrum of possible T cell responses to recognition of cognate antigen during infection, adding immense diversity to the immune system’s response to pathogens.

CD5 instructs extrathymic regulatory T cell development in response to self and tolerizing antigens

Self-reactive T cells can escape thymic deletion and therefore some of these potentially autoaggressive T cells need to convert into regulatory T (Treg) cells to help control responses against self. However, it remains unknown how peripheral self-reactive T cells are specifically instructed to become Treg cells. We report that CD5, whose expression is upregulated in T cells by self and tolerizing antigens in the thymus and periphery, governed extrathymic Treg cell development. CD5 modified effector cell-differentiating signals that inhibit Treg cell induction. Treg cell conversion of Cd5(-/-) and CD5(lo) T cells was inhibited by even small amounts of interleukin-4 (IL-4), IL-6, and interferon-γ (IFN-γ) produced by bystander lymphocytes, while CD5(hi) T cells resisted this inhibition of Treg cell induction. Our findings further revealed that CD5 promoted Treg cell induction by blocking mechanistic target of rapamycin (mTOR) activation. Therefore CD5 instructs extrathymic Treg cell development in response to self and tolerizing antigens.

T-cell modulatory properties of CD5 and its role in antitumor immune responses

The destruction of tumor cells by the immune system is under the control of positive and negative receptors that tightly regulate T-cell effector functions. The T-cell receptor (TCR) inhibitory molecule CD5 critically contributes to the regulation of antitumor immune responses. Indeed, the modulation of CD5 within the tumor microenvironment corresponds to a strategy adopted by tumor-specific cytotoxic T lymphocytes (CTLs) to optimize their cytotoxic and cytokine secretion functions. In this review, we provide insights into the immunobiology of CD5 and its role in regulating antitumor CD8 T-cell responses, and suggest the possibility of targeting CD5 to improve the efficacy of current immunotherapeutic approaches against cancer.

CD5-dependent CK2 activation pathway regulates threshold for T cell anergy

CD5 activates casein kinase 2 (CK2), a serine/threonine kinase that constitutively associates with the CK2-binding domain at the end of its cytoplasmic tail. To determine the physiological significance of CD5-dependent CK2 activation in T cells, we generated a knock-in mouse that expresses a CD5 protein containing a microdeletion with selective inability to interact with CK2 (CD5ΔCK2BD). The levels of CD5 on developing and mature T cell populations from CD5ΔCK2BD mice and CD5 wild-type (WT) mice were similar. The thymus of CD5ΔCK2BD mice contained fewer double-positive thymocytes than did that of both CD5WT and CD5 knockout (KO) mice, although the numbers of all other immature and mature T cell populations were unaltered. CD5ΔCK2BD T cells hypoproliferated and exhibited enhanced activation-induced cell death when stimulated with anti-CD3 or cognate peptide in comparison with CD5WT T cells. We also found that functional CD5-dependent CK2 signaling was necessary for efficient differentiation of naive CD4+ T cells into Th2 and Th17 cells, but not Th1 cells. We previously showed that experimental autoimmune encephalomyelitis (EAE) in CD5KO mice was less severe and delayed in onset than in CD5WT mice. Remarkably, CD5ΔCK2BD mice recapitulated both EAE severity and disease onset of CD5KO mice. Increasing the immunization dose of myelin oligodendrocyte glycoprotein 35-55 peptide, a model that mimics high-dose tolerance, led to decreased severity of EAE in CD5WT mice but not in CD5KO or CD5ΔCK2BD mice. This property was recapitulated in in vitro restimulation assays. These results demonstrate that CD5-CK2 signaling sets the threshold for T cell responsiveness and is necessary for efficient generation of Th2 and Th17 cells.

Programmed cell death 1 suppresses B-1b cell expansion and long-lived IgG production in response to T cell-independent type 2 antigens

B-1b cells play a key role in producing Abs against T cell-independent type 2 Ags. However, the factors regulating Ab production by this unique B cell subset are not well understood. In this study, a detailed analysis of the B cell response to 2,4,6-trinitrophenol (TNP)-Ficoll was performed using normal mice. TNP-Ficoll delivered i.p. or i.v. induced rapid Ag-specific B-1b cell activation, expansion, isotype switching, and plasmablast/plasma cell differentiation. Ag-specific B-1b cell numbers peaked at day 5 and then gradually declined in the spleen but remained elevated in the peritoneal cavity beyond 40 d postimmunization. In addition to expressing CD43, CD44, and CD86, Ag-activated B-1b cells transiently expressed programmed cell death 1 (PD-1), which functionally suppressed BCR-induced B-1b cell in vitro proliferation when additional costimulatory signals were lacking. Inhibiting PD-1:PD-1 ligand interactions during TNP-Ficoll immunization significantly enhanced Ag-specific B-1b cell expansion and the frequency of IgG isotype switching and plasmablast/plasma cell differentiation. Remarkably, PD-1 mAb blockade during the first week following immunization resulted in significantly increased numbers of both splenic and bone marrow Ag-specific IgG3-secreting cells, but not IgM-secreting cells, at both early (day 5) and late (week 6) time points. Moreover, Ag-specific serum IgG3 levels, as well as IgG2c, IgG2b, and IgA levels, remained significantly elevated in PD-1 mAb-treated mice relative to control Ab-treated mice for ≥6 wk postimmunization. Thus, PD-1:PD-1 ligand interactions occurring shortly after initial T cell-independent type 2 Ag encounter play a critical role in suppressing Ag-specific B-1b cell expansion and the development of long-term IgG-producing bone marrow and spleen cells.

Rescue of tumor-infiltrating lymphocytes from activation-induced cell death enhances the antitumor CTL response in CD5-deficient mice

The CD5 coreceptor is expressed on all T cells and on the B1a B cell subset. It is associated with TCR and BCR, and modulates intracellular signals initiated by both Ag receptor complexes. Human CD5 contributes to regulation of the antitumor immune response and susceptibility of specific CTL to activation-induced cell death (AICD) triggered by the tumor. In this study, we compared the T cell response to the B16F10 melanoma engrafted into CD5-deficient and wild-type C57BL/6 mice. Compared with wild-type mice, CD5 knockout animals displayed delayed tumor growth, associated with tumor infiltration by T cell populations exhibiting a more activated phenotype and enhanced antitumor effector functions. However, control of tumor progression in CD5(-/-) mice was transient due to increased AICD of CD8(+) tumor-infiltrating T lymphocytes. Remarkably, in vivo protection of T cells from TCR-mediated apoptosis by an adenovirus engineered to produce soluble Fas resulted in a dramatic reduction in tumor growth. Our data suggest that recruitment of tumor-specific T cells in the tumor microenvironment occurs at early stages of cancer development and that tumor-mediated AICD of tumor-infiltrating T lymphocytes is most likely involved in tumor escape from the immune system.

A conserved enhancer element differentially regulates developmental expression of CD5 in B and T cells

We previously identified an enhancer element upstream of the mouse cd5 gene that was required in reporter assays for the induction of cd5 promoter activity by BCR cross-linking. This element is highly conserved in placental mammals. To determine its physiological role, we have now generated mice with a targeted deletion of the enhancer. The result is the loss of CD5 expression in peritoneal and splenic B-1a cells of adult mice and an inability to induce CD5 by cross-linking of the BCR on splenic B-2 cells. Surprisingly, CD5 expression on B-1a cells of neonatal mice was only minimally compromised. Cd5 enhancer deletion also had only a modest effect on CD5 expression in the T lineage. Thus, this enhancer provides age- and tissue-specific regulation of CD5 expression and is an example of the utilization of different modes of regulation of expression in T and B cells.

CD19 signaling is impaired in murine peritoneal and splenic B-1 B lymphocytes

B-1 cells reside predominantly within the coelomic cavities, tonsils, Peyer's patches, spleen (a minor fraction - approximately 5%) and are absent in the lymph nodes. They are the primary sources of natural IgM in the body. B-1 cells express polyreactive B cell receptors (BCRs) that cross react with self-antigens and are thus implicated in auto-immune disorders. Previously, we reported that peritoneal B-1 cells are deficient in CD19-mediated intracellular signals leading to Ca(2+) mobilization. Here, we find that splenic B-1 cells, like peritoneal B-1 cells, are defective in Ca(2+) release upon B cell activation by co-cross-linking BCR and CD19. In the absence of extracellular sources of Ca(2+), intracellular Ca(2+) flux is similar between B-1 and B-2 cells. Moreover, the intracellular component of Ca(2+) release in both subsets of B cells is mostly PI3K dependent. BCR and CD19 co-cross-linking activates Akt, a key mediator of survival and proliferation signals downstream of PI3K in splenic B-2 cells. Splenic B-1 cells, on the other hand, do not phosphorylate Akt (S473) upon similar treatment. Furthermore, BCR+CD19 cross-linking induced phosphorylation of JNK is much reduced in splenic B-1 cells. In contrast, B-1 cells exhibited increased levels of constitutively active pLyn which appears to have an inhibitory role. The CD19 induced Ca(2+) response and BCR induced proliferation response were restored by a partial inhibition of pLyn with Src kinase specific inhibitors. These findings suggest a defect in CD19-mediated signals in both peritoneal and splenic B-1 B lymphocytes, which is in part, due to higher levels of constitutively active Lyn.

Removal of the BiP-retention domain in Cmicro permits surface deposition and developmental progression without L-chain

Nascent, full length, immunoglobulin (Ig) heavy (H)-chains are post-translationally associated with H-chain-binding protein (BiP or GRP78) in the endoplasmic reticulum (ER). The first constant (C) domain, CH1 of a C gene (Cmu, Cgamma, Calpha), is important for this interaction. The contact is released upon BiP replacement by conventional Ig light (L)-chain (kappa or lambda). Incomplete or mutated H-chains with removed variable (VH) and/or C(H)1 domain, as found in H-chain disease (HCD), can preclude stable BiP interaction. Progression in development after the preB cell stage is dependent on surface expression of IgM when association of a micro H-chain with a L-chain overcomes the retention by BiP. We show that IgM lacking the BiP-binding domain is displayed on the cell surface and elicits a signal that allows developmental progression even without the presence of L-chain. The results are reminiscent of single chain Ig secretion in camelids where developmental processes leading to the generation of fully functional H-chain-only antibodies are not understood. Furthermore, in the mouse the largest secondary lymphoid organ, the spleen, is not required for H-chain-only Ig expression and the CD5 survival signal may be obsolete for cells expressing truncated IgM.

Murine models of immunodeficiency and autoimmune disease

Genetically determined murine immunodeficiency states are useful for understanding the function of specific immune-system genes and cellpopulations. In addition, certain immunodeficient strains may be exploited as hosts for foreign tumors or immune cells. The more commonly used immunodeficiency models are described in this appendix. Not included are strains better known for primary neurological or neuromuscular abnormalities or for defective osteoclast function. Many of the recently described immune-deficient "knockout" strains are described, including cytokine and cytokine receptor knockout strains. The most widely studied murine strains for autoimmune disease and experimental autoreactivity are also listed.

Three-dimensional solution structure and conformational plasticity of the N-terminal scavenger receptor cysteine-rich domain of human CD5

The lymphocyte receptor CD5 influences cell activation by modifying the strength of the intracellular response initiated by antigen engagement. Regulation through CD5 involves the interaction of one or more of its three scavenger receptor cysteine-rich domains present in the extracellular region. Here, we present the 3D solution structure of a non-glycosylated double mutant of the N-terminal domain of human CD5 expressed in Escherichia coli (eCD5d1m), which has enhanced solubility compared to the non-glycosylated wild-type (eCD5d1). In common with a glycosylated form expressed in Pichia pastoris, the [(15)N,(1)H]-correlation spectra of both eCD5d1 and eCD5d1m exhibit non-uniform temperature-dependent signal intensities, indicating extensive conformational fluctuations on the micro-millisecond timescale. Although approximately one half of the signals expected for the domain are absent at 298 K, essentially complete resonance assignments and a solution structure could be obtained at 318 K. Because of the sparse nature of the experimental restraint data and the potentially important contribution of conformational exchange to the nuclear Overhauser effect peak intensity, we applied inferential structure determination to calculate the eCD5d1m structure. The inferential structure determination ensemble has similar features to that obtained by traditional simulated annealing methods, but displays superior definition and structural quality. The eCD5d1m structure is similar to other members of the scavenger receptor cysteine-rich superfamily, but the position of the lone alpha helix differs due to interactions with the unique N-terminal region of the domain. The availability of an experimentally tractable form of CD5d1, together with its 3D structure, provides new tools for further investigation of its function within intact CD5.

CD5-CK2 binding/activation-deficient mice are resistant to experimental autoimmune encephalomyelitis: protection is associated with diminished populations of IL-17-expressing T cells in the central nervous system

Regulating the differentiation and persistence of encephalitogenic T cells is critical for the development of experimental autoimmune encephalomyelitis (EAE). We reported recently that CD5 has an engagement-dependent prosurvival activity in T cells that played a direct role in the induction and progression EAE. We predicted that CD5 regulates T cell apoptosis/survival through the activation of CK2, a prosurvival serine/threonine kinase that associates with the receptor. To test this hypothesis, we generated mice expressing CD5 with the inability to bind and activate CK2 and assessed their susceptibility to EAE. We found mice deficient in CD5-CK2 signaling pathway were mostly resistant to the development of EAE. Resistance to EAE was associated with a dramatic decrease in a population of effector infiltrating Th cells that coexpress IFN-gamma and IL-17 and, to a lesser extent, cells that express IFN-gamma or IL-17 in draining lymph nodes and spinal cords. We further show that T cells deficient in CD5-CK2 signaling hyperproliferate following primary stimulation; however, following restimulation, they rapidly develop nonresponsiveness and exhibit elevated activation-induced cell death. Our results provide a direct role for CD5-CK2 pathway in T cell activation and persistence of effector T cells in neuroinflammatory disease. This study predicts that targeting of IFN-gamma(+)/IL-17(+) infiltrating Th cells will be useful for the treatment of multiple sclerosis and other systemic autoimmune diseases.

Regulation of T cell development by the deubiquitinating enzyme CYLD

T cell receptor signaling is essential for the generation and maturation of T lymphocyte precursors. Here we identify the deubiquitinating enzyme CYLD as a positive regulator of proximal T cell receptor signaling in thymocytes. CYLD physically interacted with active Lck and promoted recruitment of active Lck to its substrate, Zap70. CYLD also removed both Lys 48- and Lys 63-linked polyubiquitin chains from Lck. Because of a cell-autonomous defect in T cell development, CYLD-deficient mice had substantially fewer mature CD4(+) and CD8(+) single-positive thymocytes and peripheral T cells.

Thymic selection can compensate for mutations affecting T cell activation and generate a normal T cell repertoire in mutant mice

Thymic selection adjusts the reactivity of the peripheral T cell repertoire to maximize recognition of pathogens and minimize stimulation by innocuous substances and self-antigen. The study of molecules implicated in T cell activation often involves the generation of knockout (-/-) mice. In many instances, knockout animals display revealing phenotypes. But should a lack of phenotype be interpreted as a lack of function? Bcl-xgamma was shown previously to affect T cell activation in vitro, and here we note that overexpression of this molecule increases cell cycling after T cell receptor ligation by antibody. It was therefore surprising that Bcl-xgamma(-/-), Bcl-xgamma transgenic, and WT T cells displayed similar levels of sensitivity to antigen according to ex vivo stimulation. Bcl-xgamma could be demonstrated to influence competitiveness and selection of thymocytes in a manner that counteracted the effects of Bcl-xgamma mutation on T cell activation. These findings suggest that thymic selection can overcome genetic defects in T cell activation to generate a T cell repertoire of normal reactivity.

Normal B-1a cell development requires B cell-intrinsic NFATc1 activity

B-1a cells, an anatomically, phenotypically, and functionally distinct subset of B cells that produce the bulk of natural serum IgM and much of gut-associated IgA, are an important component of the early response to pathogens. Because the induced expression of CD5, a hallmark of B-1a cells, requires a nuclear factor of activated T cells (NFAT)-dependent enhancer, we examined the role of NFAT transcription factors in B-1a development. Here we show that the B-1a compartment is normal in mice lacking NFATc2 but essentially absent in mice lacking NFATc1. Loss of NFATc1 affects both peritoneal and splenic B-1a cells. Because there is a loss of B-1 cells defined by markers other than CD5, NFATc1 is not required simply for CD5 expression on B-1a cells. Using mixed-allotype chimeras and retroviral-mediated gene transduction we show that the requirement for NFATc1 is B cell-intrinsic. We also demonstrate that NFATc1 protein expression is elevated approximately 5-fold in B-1a cells compared with B-2 cells. This is the first definitive demonstration of a B cell-intrinsic function for an NFAT family transcription factor.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Absence of CD5 dramatically reduces progression of pulmonary inflammatory lesions in SHP-1 protein-tyrosine phosphatase-deficient 'viable motheaten' mice

Mice homozygous for the viable motheaten (Hcph(me-v)) mutation are deficient in SHP-1 protein-tyrosine phosphatase, resulting in severe systemic autoimmunity and immune dysfunction. A high percentage of B-cells in viable motheaten mice express the cell surface glycoprotein CD5, in contrast to wild type mice that express CD5 on only a small percentage of B-cells. CD5(+) B-cells have been associated with autoantibody production. To determine the role of CD5 in the development of the inflammatory disease in me(v)/ me(v) mice, we created a stock of CD5(null)me(v)/ me(v) mice. The longevity of CD5(null)me(v)/ me(v) mice was increased 69% in comparison to me(v)/ me(v) mice on a similar (B6;129) background. The increased lifespan was associated with a marked reduction in pulmonary inflammation. Flow cytometry analysis of spleen cells from CD5(null)me(v)/ me(v) mice at 9-12 weeks of age revealed significant decreases in percentages of IgM/B220 double positive B-cells, Mac-1/Gr-1 double positive cells and CD4(+) T-cells compared with me(v)/ me(v) mice. CD5(null)me(v)/ me(v) mice also had significantly lower serum IgM levels in comparison to me(v)/ me(v) mice. Study of CD5(null)me(v)/ me(v) mice may provide further insight into the role of CD5 in cell signaling and may help explain the observed association of CD5(+) B-cells with autoimmune disease.

TOX: an HMG box protein implicated in the regulation of thymocyte selection

In the thymus, pre-T cell receptor (pre-TCR)--mediated signaling and then TCR-mediated signaling initiate changes in gene expression that result in the maturation of CD4 and CD8 lineage T cells from common precursors. Using gene chip technology, we isolated a murine gene, designated Tox, that encodes a member of the HMG (high-mobility group) box family of DNA-binding proteins. TOX expression is up-regulated by both pre-TCR and TCR activation of immature thymocytes but not by TCR activation of mature naïve T cells. Transgenic mice that express TOX show expanded CD8+ and reduced CD4+ single positive thymocyte subpopulations. We present evidence here that this phenotype results from a perturbation in lineage commitment due to reduced sensitivity to TCR-mediated signaling. This molecular marker of thymic selection events may therefore play a role in establishing the activation threshold of developing T cells and patterning changes in gene expression.

Age-dependent increase of peritoneal B-1b B cells in SCID mice

The impact of increasing age upon immunoglobulin production and B-lymphocyte generation in "leaky" severe combined immune-defective (SCID) mice was examined by enzyme-linked immunosorbent assay and flow cytometry. By 1 year of age, the mice had normal numbers of B cells in their peritoneal cavity, while their spleen had very few immunoglobulin M-positive (IgM+) cells. The majority of B cells expressed the CD11b marker characteristic of the B-1b subset. B-1a (CD5+) cells were present at a lower frequency and B-2 cells were absent. The frequency of mice producing detectable immunoglobulin increased with age, and isotype diversity within individual mice was variable. IgM production was most frequently observed followed by IgG3 and IgG2a, then IgG1, and finally IgA. The selective persistence of the B-1 B-cell subset in the peritoneal cavity of aging SCID mice is a natural model for the study of those genetic and environmental influences that determine lymphocyte longevity.

Sensory adaptation in naive peripheral CD4 T cells

T cell receptor interactions with peptide/major histocompatibility complex (pMHC) ligands control the selection of T cells in the thymus as well as their homeostasis in peripheral lymphoid organs. Here we show that pMHC contact modulates the expression of CD5 by naive CD4 T cells in a process that requires the continued expression of p56(lck). Reduced CD5 levels in T cells deprived of pMHC contact are predictive of elevated Ca(2)+ responses to subsequent TCR engagement by anti-CD3 or nominal antigen. Adaptation to peripheral pMHC contact may be important for regulating naive CD4 T cell responsiveness.

The Src-like adaptor protein downregulates the T cell receptor on CD4+CD8+ thymocytes and regulates positive selection

In this report, we show that the Src-like adaptor protein (SLAP) plays an important role in thymocyte development. SLAP expression is developmentally regulated; it is low in CD4-CD8- thymocytes, it peaks in the CD4+CD8+ subset, and it decreases to low levels in more mature cells. Disruption of the SLAP gene leads to a marked upregulation of TCR and CD5 expression at the CD4+CD8+ stage. The absence of SLAP was also developmentally significant because it enhanced positive selection in mice expressing the DO11.10 transgenic T cell receptor. Moreover, SLAP deletion at least partially rescued the development of ZAP-70-deficient thymocytes. These results demonstrate that SLAP participates in a novel mechanism of TCR downregulation at the CD4+CD8+ stage and regulates positive selection.

A low affinity TCR ligand restores positive selection of CD8+ T cells in vivo

The T cell repertoire is shaped by the processes of positive and negative selection. During development, the TCR binds self peptide-MHC complexes in the thymus, and the kinetics of this interaction are thought to determine the thymocyte's fate. For development of CD8(+) T cells, the data supporting such a model have been obtained using fetal thymic organ culture. To confirm the fidelity of this model in vivo, we studied development of OT-I TCR-transgenic mice that expressed different individual K(b) binding peptides in thymic epithelial cells under the control of the human keratin 14 promoter. We used a system that allowed TAP-independent expression of the peptide-MHC complex, such that the ability of given peptides to restore positive selection in TAP(o) mice could be assessed. We found that transgenic expression of a TCR antagonist peptide (E1) in vivo efficiently restored positive selection of OT-I T cells in TAP(o) mice. An unrelated transgenic peptide (SIY) did not restore selection of OT-I T cells, nor did the E1-transgenic peptide restore selection of an unrelated receptor (2C), showing that positive selection is peptide specific in vivo, as observed in organ cultures. Neither E1 nor SIY transgenes increased the polyclonal CD8 T cell repertoire size in non-TCR-transgenic animals, arguing that single class I binding peptides do not detectably affect the size of the CD8 T cell repertoire when expressed at low levels. We also observed that OT-I T cells selected in TAP(o)-E1 mice were functional in their response to Ag; however, there was a lag in this response, suggesting that the affinity of the TCR interaction with MHC-self peptide can result in fine-tuning of the T cell response.

Fine tuning of TCR signaling by CD5

Current data indicate that CD5 functions as an inhibitor of TCR signal transduction. Consistent with this role, thymocyte selection in TCR transgenic/CD5(-/-) mice is altered in a manner suggestive of enhanced TCR signaling. However, the impact of CD5 deletion on thymocyte selection varies depending on the transgenic TCR analyzed, ranging from a slight to a marked shift from positive toward negative selection. An explanation for the variable effect of CD5 on selection is suggested by the observation that CD5 surface expression is regulated by TCR signal intensity during development and CD5 surface levels on mature thymocytes and T cells parallel the avidity of the positively selecting TCR/MHC/ligand interaction. In this study, we generated mice that overexpress CD5 during thymocyte development (CD5-tg), and then examined the effect of CD5 overexpression or CD5 deletion (CD5(-/-)) on selection of thymocytes that express the same TCR transgenes. The results demonstrate that the effect on thymocyte selection of altering CD5 expression depends on the avidity of the selecting interaction and, consequently, the level of basal (endogenous) CD5 surface expression. Substitution of endogenous CD5 with a transgene encoding a truncated form of the protein failed to rescue the CD5(-/-) phenotype, demonstrating that the cytoplasmic domain of CD5 is required for its inhibitory function. Together, these results indicate that inducible regulation of CD5 surface expression during thymocyte selection functions to fine tune the TCR signaling response.

Cloning of a novel scavenger receptor cysteine-rich type I transmembrane molecule (M160) expressed by human macrophages

We report the cloning of a novel human type I cell surface Ag mainly expressed by macrophages. The primary structure was established by molecular cloning, which yielded a 4579-bp cDNA sequence encoding a polypeptide chain of 1453 amino acid residues with 16 potential N:-glycosylation sites. We designated this molecule M160. The domain organization features 12 scavenger receptor cysteine-rich domains followed by a transmembrane region and a cytoplasmic domain that occurs in two forms, a predominant form (M160-alpha) of 71 residues and an alternatively spliced form (M160-ss) of 39 residues. M160-alpha contains three possible phosphorylation sites, which are lost in the alternatively spliced form. RT-PCR analyses showed M160 to be expressed by alveolar macrophages and by the monocyte cell lines HL60, U937, and THP1, but not by Jurkat or Raji cells. Stimulation of U937 cells with phorbol ester resulted in an increased expression of M160 from day 5 onward. RT-PCR analysis of 19 different human tissues showed signals for M160-alpha of varying intensity in all tissues, whereas M160-ss was confined to the spleen. We conclude that M160 is a new member of the scavenger receptor cysteine-rich superfamily expressed by the monocyte/macrophage cell lineage.

The toll-like receptor protein RP105 regulates lipopolysaccharide signaling in B cells

The susceptibility to infections induced by Gram-negative bacteria is largely determined by innate immune responses to bacteria cell wall lipopolysaccharide (LPS). The stimulation of B cells by LPS enhances their antigen-presenting capacity and is accompanied by B cell proliferation and secretion of large quantities of LPS-neutralizing antibodies. Similar to macrophages and neutrophils, the LPS-induced activation of B cells is dependent on Toll-like receptor (TLR)4. Here, we demonstrate that the responses of B cells to LPS are also regulated by another TLR protein, RP105, which is predominantly expressed on mature B cells in mice and humans. The analysis of mice homozygous for the null mutation in the RP105 gene revealed impaired proliferative and humoral immune responses of RP105-deficient B cells to LPS. Using originally LPS-unresponsive Ba/F3 cells expressing exogenous TLR4 and RP105, we demonstrate the functional cooperation between TLR4 and RP105 in LPS-induced nuclear factor kappaB activation. These data suggest the existence of the TLR4-RP105 signaling module in the LPS-induced B cell activation.

Cloning and functional characterization of the early-lymphocyte-specific Pb99 gene

The Pb99 gene is specifically expressed in pre-B cells and thymocytes and not in mature B and T cells or nonlymphoid tissues, implying that it may function in early lymphoid development. We have previously described the cloning of an incomplete cDNA for Pb99. Here we report the isolation of full-length cDNAs and genomic clones for the murine Pb99 gene and the mapping of its location to mouse chromosome 8. Sequence analyses of different Pb99 cDNA clones suggest that there may be at least three forms of the Pb99 protein generated by differential processing of the Pb99 transcript. The cDNA with the longest open reading frame encodes a putative protein that has seven hydrophobic domains similar to those of seven membrane-spanning proteins, such as the classical G protein-coupled receptors. To directly address the role of the Pb99 protein in lymphoid development, Pb99-deficient mice were generated by gene targeting, and lymphocyte development in these mice was analyzed.

Reduced interleukin-1 responsiveness in immune system and central nervous system of inbred polydipsic (STR/N) mice

Inbred polydipsic mice (STR/N strain) have primary polydipsia. The previous studies found abnormalities in the central nervous system (CNS), especially in the hypothalamus and circumventricular organ. As a part of pursuing to find the cause of the polydipsia, we investigated immunological characteristics of STR/N mice, using the ICR strain of mice as control. Their thymic subset cells showed that CD4+CD8+ double positive cells were increased, CD4+ single positive cells were decreased and CD5 expression was deficient, compared to ICR mice. T cell proliferative response and interleukin (IL)-2 production caused by IL-1beta stimulation were reduced in STR/N mice than those in the ICR mice. In in vivo studies the degree of thymic atrophy and the increases in serum level of ACTH and corticosterone induced by intraperitoneal IL-1beta injection were much less in STR/N mice than those in controls. Furthermore, adipsic response also induced by IL-1beta injection was greatly reduced compared to their control mice. All these results suggest that the responsiveness to IL-1 is impaired both in the immune system and the CNS of STR/N mice.

CD5 maintains tolerance in anergic B cells

Clonal anergy of autoreactive B cells is a key mechanism regulating tolerance. Here, we show that anergic B cells express significant surface levels of CD5, a molecule normally found on T cells and a subset of B-1 cells. Breeding of the hen egg lysozyme (HEL) transgenic model for B cell anergy onto the CD5 null background resulted in a spontaneous loss of B cell tolerance in vivo. Evidence for this included elevated levels of anti-HEL immunoglobulin M (IgM) antibodies in the serum of CD5(-/-) mice transgenic for both an HEL-specific B cell receptor (BCR) and soluble lysozyme. "Anergic" B cells lacking CD5 also showed enhanced proliferative responses in vitro and elevated intracellular Ca(2+) levels at rest and after IgM cross-linking. These data support the hypothesis that CD5 negatively regulates Ig receptor signaling in anergic B cells and functions to inhibit autoimmune B cell responses.

Molecular cloning, genomic organization and cell-binding characteristics of mouse Spalpha

Several group B scavenger receptor cysteine-rich (SRCR) proteins have been shown to function as modulators in the immune response. Recently, we reported the cloning of a new member of this family, human Spalpha (hSpalpha). Herein we report the cloning and characterization of the mouse homologue of hSpalpha. Like its human counterpart, mouse Spalpha (mSpalpha), is a secreted protein containing three SRCR domains. Most lymphoid tissues express RNA transcripts encoding mSpalpha. Characterization of a genomic clone encoding the mature mSpalpha protein showed that each of the SRCR domains of mSpalpha is encoded by a single exon. Comparison of the sequence of mSPalpha with those of other published proteins indicates that it is the same as the recently reported protein named AIM (apoptosis inhibitor expressed by macrophages). Cell-binding studies with a mSpalpha immunoglobulin (mSpalpha-Rgamma) fusion protein indicated that mSpalpha is capable of binding to spleen-derived CD19+ B cells and minimally to peritoneal cavity-derived CD19+ B cells but not to peripheral blood-derived B cells. Spleen-derived CD3+ T cells also bound mSpalpha-Rgamma; however, no binding was observed to either peripheral blood mononuclear cells or peritoneal cavity-derived CD3+ T cells. The mSpalpha-Rgamma fusion protein was also shown to bind to the mouse cell lines WEHI3 (monocytic) and EL-4 (thymoma, T cell). The cloning of cDNA and genomic clones encoding mSpalpha and the identification of cells expressing a putative mSpalpha receptor(s) should facilitate in vivo studies designed to investigate the function of Spalpha in the immune compartment.

Negative Regulation of CD4 Lineage Development and Responses by CD5

CD5 deficiency results in a hyper-responsive phenotype to Ag receptor stimulation. Here we show that the development and responses of CD4 lineage T cells are regulated by the function of CD5. Thymocytes expressing the I-Ad-restricted DO11.10 TCR undergo abnormal selection without CD5. In H-2d mice, the absence of CD5 causes deletion of double-positive thymocytes, but allows for efficient selection of cells expressing high levels of the DO11.10 clonotype. By contrast, there is enhanced negative selection against the DO11.10 clonotype in the presence of I-Ab. T cell hybridomas and DO11.10 T cells are more responsive to TCR stimulation in the absence of CD5. Such hypersensitivity can be eliminated by expression of wild-type CD5, but not by a form of CD5 that lacks the cytoplasmic tail. Finally, CD5 deficiency partially suppresses the block of CD4 lineage development in CD4-deficient mice. Taken together, the data support a general role for CD5 as a negative regulator of Ag receptor signaling in the development and immune responses of CD4 lineage T cells.

Negative regulation of antigen receptor-mediated signaling by constitutive association of CD5 with the SHP-1 protein tyrosine phosphatase in B-1 B cells

CD5, a membrane-associated glycoprotein, has been shown to negatively regulate antigen receptor-mediated growth responses in peritoneal B lymphocytes, thymocytes and mature T cells. The CD5-expressing peritoneal B cells (B-1) that are normally unresponsive to B cell receptor (BCR)-mediated growth signals mount a proliferative response to BCR cross-linking if the CD5 gene is deleted or if the CD5 molecule is sequestered away from the BCR. SHP-1, a cytosolic protein tyrosine phosphatase, has also been implicated in the negative regulation of antigen receptor-mediated signaling. The present study shows that SHP-1 is constitutively associated with the BCR in B-1 cells. This association is mediated in part by CD5, as it is reduced substantially after antigen receptor ligation in CD5(-/-) B-1 cells, and upon sequestration of CD5 from the antigen receptor complexes in wild-type B-1 cells. Prior cross-linking of CD5 also restores a normal calcium mobilization response as well as NF-kappaB activation in B-1 cells. These data support a model whereby CD5 negatively regulates antigen receptor-mediated growth signals by recruiting SHP-1 into the BCR complex in B-1 cells.

A Novel Role for HEB Downstream or Parallel to the Pre-TCR Signaling Pathway During αβ Thymopoiesis

TCR gene rearrangement and expression are central to the development of clonal T lymphocytes. The pre-TCR complex provides the first signal instructing differentiation and proliferation events during the transition from CD4−CD8−TCR− double negative (DN) stage to CD4+CD8+ double positive (DP) stage. How the pre-TCR signal leads to downstream gene expression is not known. HeLa E-box binding protein (HEB), a basic helix-loop-helix transcription factor, is abundantly detected in thymocytes and is thought to regulate E-box sites present in many T cell-specific gene enhancers, including TCR-α, TCR-β, and CD4. Targeted disruption of HEB results in a 5- to 10-fold reduction in thymic cellularity that can be accounted for by a developmental block at the DN to DP stage transition. Specifically, a dramatic increase in the CD4low/−CD8+CD5lowHSA+TCRlow/− immature single positive population and a concomitant decrease in the subsequent DP population are observed. Adoptive transfer test shows that this defect is cell-autonomous and restricted to the αβ T cell lineage. Introduction of an αβ TCR transgene into the HEBko/ko background is not sufficient to rescue the developmental delay. In vivo CD3 cross-linking analysis of thymocytes indicates that TCR signaling pathway in the HEBko/ko mice appears intact. These findings suggest an essential function of HEB in early T cell development, downstream or parallel to the pre-TCR signaling pathway.

An influence of CD5 on the selection of CD4-lineage T cells

Combining CD5-null, MHC-deficient and lineage-specific reporter animals, we have investigated the influence of CD5 on positive selection and the choice of CD4- versus CD8-lineage commitment on broad populations of thymocytes. CD5 has no obvious quantitative effect in wild-type mice. In mice lacking MHC class II molecules, however, increased numbers of transitional, class I-selected CD4+ CD8(int) CD3(hi) cells were positively selected in the absence of CD5. Importantly, they were committed to the CD4 lineage. Our results indicate that CD5 negatively regulates the differentiation of CD4-committed cells in suboptimal conditions, thus perhaps serving to tighten the correlation between restriction of the TCR and lineage choice.