CD47 regulates collagen I-induced cyclooxygenase-2 expression and intestinal epithelial cell migration

Third-Generation Anti-CD47-Specific CAR-T Cells Effectively Kill Cancer Cells and Reduce the Genes Expression in Lung Cancer Cell Metastasis

CD47 is a cell surface glycoprotein molecule, belonging to the immunoglobulin superfamily, binding to various proteins including integrins, thrombospondin-1, and signal regulatory protein α (SIRPα). CD47 is an important tumor antigen for the development and progression of various cancers. This study designed the chimeric antigen receptor T-cell (CAR-T) to bind to the CD47 to inhibit the expression of CD47. We used the complementarity-determining regions (CDRs) of the B6H12 mouse antibody grafted onto the IgG1 framework to create the humanized single-chain variable fragment (scFv) with linker (G4S)x3. scFv was used to design the chimeric antigen receptor with the structure CD8signal-CD47scFv-CD8a hinge-CD4TM-CD28-41BB-CD3ζ, which was then transformed into T lymphocytes by the lentivirus to create third generation of CAR-T. Results revealed that the new CAR-T cells efficiently killed A549 cancer cells. CAR-T inhibited the expression of genes involved in metastasis and invasion of cells A549 including beta actin, calreticulin, and cyclooxygenase 2 at mRNA levels.

CD47 (Cluster of Differentiation 47)

CD47, also known as integrin-associated protein, is a constitutively and ubiquitously expressed transmembrane receptor. CD47 is conserved across amniotes including mammals, reptiles, and birds. Expression is increased in many cancers and, in non-malignant cells, by stress and with aging. The up-regulation of CD47 expression is generally epigenetic, whereas gene amplification occurs with low frequency in some cancers. CD47 is a high affinity signaling receptor for the secreted protein thrombospondin-1 (THBS1) and the counter-receptor for signal regulatory protein-α (SIRPA, SIRPα) and SIRPγ (SIRPG). CD47 interaction with SIRPα serves as a marker of self to innate immune cells and thereby protects cancer cells from phagocytic clearance. Consequently, higher CD47 correlates with a poor prognosis in some cancers, and therapeutic blockade can suppress tumor growth by enhancing innate antitumor immunity. CD47 expressed on cytotoxic T cells, dendritic cells, and NK cells mediates inhibitory THBS1 signaling that further limits antitumor immunity. CD47 laterally associates with several integrins and thereby regulates cell adhesion and migration. CD47 has additional lateral binding partners in specific cell types, and ligation of CD47 in some cases modulates their function. THBS1-CD47 signaling in non-malignant cells inhibits nitric oxide/cGMP, calcium, and VEGF signaling, mitochondrial homeostasis, stem cell maintenance, protective autophagy, and DNA damage response, and promotes NADPH oxidase activity. CD47 signaling is a physiological regulator of platelet activation, angiogenesis and blood flow. THBS1/CD47 signaling is frequently dysregulated in chronic diseases.

Role of CD47 in Hematological Malignancies

CD47, or integrin-associated protein, is a cell surface ligand expressed in low levels by nearly all cells of the body. It plays an integral role in various immune responses as well as autoimmunity, by sending a potent "don't eat me" signal to prevent phagocytosis. A growing body of evidence demonstrates that CD47 is overexpressed in various hematological malignancies and its interaction with SIRPα on the phagocytic cells prevents phagocytosis of cancer cells. Additionally, it is expressed by different cell types in the tumor microenvironment and is required for establishing tumor metastasis. Overexpression of CD47 is thus often associated with poor clinical outcomes. CD47 has emerged as a potential therapeutic target and is being investigated in various preclinical studies as well as clinical trials to prove its safety and efficacy in treating hematological neoplasms. This review focuses on different therapeutic mechanisms to target CD47, either alone or in combination with other cell surface markers, and its pivotal role in impairing tumor growth and metastatic spread of various types of hematological malignancies.

Preclinical and Clinical Development of Therapeutic Antibodies Targeting Functions of CD47 in the Tumor Microenvironment

CD47 is a ubiquitously expressed cell surface glycoprotein that functions as a signaling receptor for thrombospondin-1 and as the counter-receptor for signal regulatory protein-α (SIRPα). Engaging SIRPα on macrophages inhibits phagocytosis, and CD47 thereby serves as a physiological marker of self. However, elevated CD47 expression on some cancer cells also protects tumors from innate immune surveillance and limits adaptive antitumor immunity via inhibitory SIRPα signaling in antigen presenting cells. CD47 also mediates inhibitory thrombospondin-1 signaling in vascular cells, T cells, and NK cells, and blocking inhibitory CD47 signaling on cytotoxic T cells directly increases tumor cell killing. Therefore, CD47 functions as an innate and adaptive immune checkpoint. These findings have led to the development of antibodies and other therapeutic approaches to block CD47 functions in the tumor microenvironment. Preclinical studies in mice demonstrated that blocking CD47 can limit the growth of hematologic malignancies and solid tumors and enhance the efficacy of conventional chemotherapy, radiation therapy, and some targeted cancer therapies. Humanized CD47 antibodies are showing promise in early clinical trials, but side effects related to enhanced phagocytic clearance of circulating blood cells remain a concern. Approaches to circumvent these include antibody preloading strategies, development of antibodies that recognize tumor-specific epitopes of CD47, SIRPα antibodies, and bivalent antibodies that restrict CD47 blockade to specific tumor cells. Preclinical and clinical development of antibodies and related biologics that inhibit CD47/SIRPα signaling are reviewed, including strategies to combine these agents with various conventional and targeted therapeutics to improve patient outcome for various cancers.

Checkpoint CD47 Function On Tumor Metastasis And Immune Therapy

The success of cancer immunotherapy on recognition checkpoints for killing cancer cells has raised a great interest of scientists in understanding new and old methods of immunotherapeutic. CD47 (cluster of differentiation 47) is a cell surface glycoprotein and widely expressed on cells, which belongs to the immunoglobulin (Ig) superfamily as a cell membrane receptor which serves in immune therapy. CD47 is an inhibitory receptor expressed on tumor cell surface and interacts with signal receptor protein-alpha (SIPR-α, also named CD172a or SHPS-1) which may escape from immune cells such as macrophage and T cells. Meanwhile, tumor cells express high CD47 protein which may secrete exosomes with high CD47 expression. The high CD47 expression-exosomes could serve the tumor metastasis process and provide transfer convenience for tumors on the microenvironment. CD47 on cancer cells can also affect the migration and invasion of cells. The high CD47 expression on tumor or CTC (circulating tumor cell) surface means the stronger migration and invasion and makes them escape from immune cells for phagocytosis such as T cells, NK (natural killer) cells and macrophage, which could be used for diagnosis and prognosis on cancer patients. Meanwhile, targeting CD47 combined with other biomarkers such as EpCAM (epithelial cell adhesion molecule), CD44, etc on cancer surface could be used to isolate CTCs from patients' blood. In terms of treatment, anti-CD47 antibody combined with another antibody such as anti-PD-L1 (programmed death-ligand 1) antibody or drugs such as rituximab, DOX or oxaliplatin also has better therapeutic effects and antitumor function to tumors. Using nanomaterials as an intermediary for CD47-related immune therapy could greatly increase the therapeutic effect and overcome multiple biological barriers for anti-CD47 antibody in vivo. In this review, we discuss the important role and the function of CD47 in tumor metastasis and also provide a reference for related research.

Epithelial CD47 is critical for mucosal repair in the murine intestine in vivo

CD47 is a ubiquitously expressed transmembrane glycoprotein that regulates inflammatory responses and tissue repair. Here, we show that normal mice treated with anti-CD47 antibodies, and Cd47-null mice have impaired intestinal mucosal wound healing. Furthermore, intestinal epithelial cell (IEC)-specific loss of CD47 does not induce spontaneous immune-mediated intestinal barrier disruption but results in defective mucosal repair after biopsy-induced colonic wounding or Dextran Sulfate Sodium (DSS)-induced mucosal damage. In vitro analyses using primary cultures of CD47-deficient murine colonic IEC or human colonoid-derived IEC treated with CD47-blocking antibodies demonstrate impaired epithelial cell migration in wound healing assays. Defective wound repair after CD47 loss is linked to decreased epithelial β1 integrin and focal adhesion signaling, as well as reduced thrombospondin-1 and TGF-β1. These results demonstrate a critical role for IEC-expressed CD47 in regulating mucosal repair and raise important considerations for possible alterations in wound healing secondary to therapeutic targeting of CD47.

The role of the transmembrane glycoprotein CD47 in healing injured intestinal mucosa is unclear. Here, the authors show that selective loss of CD47 in the murine intestinal epithelium results in defective mucosal repair after colonic wounding, with suggested impaired cell migration in vitro.

CD47: role in the immune system and application to cancer therapy

BACKGROUND

CD47 is a widely expressed cellular receptor well known for its immunoregulatory functions. By interacting with its ligands, including thrombospondin-1 (TSP-1), signal regulatory protein α (SIRPα), integrins, and SH2-domain bearing protein tyrosine phosphatase substrate-1 (SHPS-1), it modulates cellular phagocytosis by macrophages, transmigration of neutrophils and activation of dendritic cells, T cells and B cells. Ample studies have shown that various types of cancer express high levels of CD47 to escape from the immune system. Based on this observation, CD47 is currently considered as a prominent target in cancer therapy.

CONCLUSIONS

Here, we review the role of CD47 in the maintenance of immune system homeostasis. We also depict three emerging CD47-targeting strategies for cancer therapy, including the use of mimicry peptides, antibodies, and gene silencing strategies. Among these approaches, the most advanced one is the use of anti-CD47 antibodies, which enhances cancer cell phagocytosis via inhibition of the CD47-SIRPα axis. These antibodies can also achieve higher anti-cancer efficacies when combined with chemotherapy and immunotherapy and hold promise for improving the survival of patients with cancer.

CD47 Promotes Human Glioblastoma Invasion Through Activation of the PI3K/Akt Pathway

Cluster of differentiation 47 (CD47) overexpression is common in various malignancies. This study investigated whether CD47 promotes human glioblastoma invasion and, if so, the underlying mechanisms involved. CD47 expression was found to be stronger in tissues of patients with glioblastoma and in various cancer cell lines than in normal controls. CD47 downregulation via siRNA suppressed invasion in vitro, whereas CD47 overexpression through plasmid transfection exerted the opposite effect. However, overexpression or knocking down of CD47 had no effect on cell proliferation. Moreover, CD47 expression was related to Akt phosphorylation at the cellular molecular level. Suppression of Akt with a specific inhibitor impaired the invasion ability of CD47-overexpressing cells, indicating that stimulation of the PI3K/Akt pathway served as the downstream regulator of CD47-triggered invasion. These results suggest that CD47 might be a useful predictor of poor prognosis and metastasis and a potential target for treating glioblastomas.

Signalling pathways in UHRF1-dependent regulation of tumor suppressor genes in cancer

Epigenetic silencing of tumor suppressor genes (TSGs) through DNA methylation and histone changes is a main hallmark of cancer. Ubiquitin-like with PHD and RING Finger domains 1 (UHRF1) is a potent oncogene overexpressed in various solid and haematological tumors and its high expression levels are associated with decreased expression of several TSGs including p16^INK4A, BRCA1, PPARG and KiSS1. Using its several functional domains, UHRF1 creates a strong coordinated dialogue between DNA methylation and histone post-translation modification changes causing the epigenetic silencing of TSGs which allows cancer cells to escape apoptosis. To ensure the silencing of TSGs during cell division, UHRF1 recruits several enzymes including histone deacetylase 1 (HDAC1), DNA methyltransferase 1 (DNMT1) and histone lysine methyltransferases G9a and Suv39H1 to the right place at the right moment. Several in vitro and in vivo works have reported the direct implication of the epigenetic player UHRF1 in tumorigenesis through the repression of TSGs expression and suggested UHRF1 as a promising target for cancer treatment. This review describes the molecular mechanisms underlying UHRF1 regulation in cancer and discusses its importance as a therapeutic target to induce the reactivation of TSGs and subsequent apoptosis.

Interleukin-6 drives melanoma cell motility through p38α-MAPK-dependent up-regulation of WNT5A expression

Extensive research has demonstrated a tumor-promoting role of increased WNT5A expression in malignant melanoma. However, very little light has been shed upon how WNT5A expression is up-regulated in melanoma. A potential regulator of WNT5A expression is the pro-inflammatory cytokine Interleukin (IL)-6, which shares the ability of WNT5A to increase melanoma cell invasion. Here, we investigate whether IL-6 can promote melanoma cell motility through an increased expression of WNT5A. We clearly demonstrate that the WNT5A-antagonistic peptide Box5 could inhibit IL-6-induced melanoma cell migration and invasion. Furthermore, IL-6 stimulation of the human melanoma cell lines HTB63 and A375 increased the expression of WNT5A in a dose-dependent manner. To identify the signaling mechanism responsible for this up-regulation, we explored the involvement of the three main signals induced by IL-6; STAT3, Akt and ERK 1/2. Of these, only STAT3 was activated by IL-6 in the melanoma cell lines tested. However, the STAT3 inhibitor S3I-201 failed to inhibit IL-6-induced WNT5A up-regulation in HTB63 and A375 cells. Nor did STAT3 siRNA silencing affect the expression of WNT5A. In search of an alternative signaling mechanism, we detected IL-6-induced activation of p38-MAPK in HTB63 and A375 cells. The p38-MAPK inhibitor SB203580 abolished the IL-6-induced WNT5A up-regulation and blocked IL-6-induced melanoma cell invasion. The latter effect could be rescued by the addition of recombinant WNT5A. Notably, immunoprecipitation analysis revealed that only the p38α-MAPK isoform was activated by IL-6, and subsequent siRNA silencing of p38α-MAPK abolished the IL-6-induced up-regulation of WNT5A. Taken together, we demonstrate a novel link between the two melanoma pro-metastatic agents IL-6 and WNT5A explaining how IL-6 can increase melanoma cell invasion and thus promote the metastatic process. This finding provides a basis for future therapeutic intervention of melanoma progression.

Down-regulation of collagen I biosynthesis in intestinal epithelial cells exposed to indomethacin: a comparative proteome analysis

In contrast to accumulated knowledge about gastroduodenal injury associated with nonsteroidal antiinflammatory drugs (NSAIDs) such as indomethacin, small intestinal mucosal injuries have been noticed only recently, and the precise mechanism remains to be elucidated. To clarify the mechanism, we performed 2-DE on IEC-6 rat normal intestinal cells that were treated with indomethacin (200μΜ, 24h) or a vehicle control and identified 18 up-regulated and 8 down-regulated proteins through MALDI-TOF/TOF mass spectrometry. Among these proteins, collagen I and proteins involved in collagen I biosynthesis and maturation, including prolyl 4-hydroxylase subunit α1, protein disulfide isomerase A3 (PDIA3), calreticulin, and endoplasmin, were all down-regulated by indomethacin. Immunohistochemical staining of the intestinal mucosa of indomethacin-administered rats showed a decrease of collagen I on the apical surface of intestinal cells. Cell death induced by indomethacin was prominently suppressed when IEC-6 cells were grown on collagen I-coated plates. cis-4-Hydroxy-l-proline, a proline analog that inhibits collagen synthesis, depressed IEC-6 cell viability in a concentration-dependent manner. Cell death was also induced by short interfering RNA knockdown of endogenous collagen I in IEC-6 cells. In conclusion, by comparative proteome analysis, we identified down-regulation of collagen I as an important mechanism in NSAID-induced intestinal injury.

BIOLOGICAL SIGNIFICANCE

Small intestinal lesions induced by NSAIDs are of great concern in clinical settings. Various hypotheses have been proposed for the origin of these inflammatory responses, such as reduction in the blood flow, intestinal hypermotility, abnormal intestinal mucosal permeability, mitochondrial dysfunction, and reactive oxygen species, many of which are related to the inhibition of prostaglandin synthesis. However, the precise mechanism is yet to be known. The cellular process of the lesions must involve up- and down-regulations of a large number of proteins and complex interactions between them. To elucidate it, global and systematic identification of the proteins in intestinal cells affected by NSAIDs is essential. We found that the proteins exhibiting reduced expression by indomethacin treatment are collagen I and the proteins involved in collagen I synthesis and maturation. Consistent with this, immunohistochemical analysis showed that the indomethacin-treated rat intestinal mucosal cells exhibits decreased collagen I expression on its apical surface. Furthermore, the cell-protective effect of collagen on intestinal mucosal cells was demonstrated by the use of a collagen-synthesis inhibitor, short interfering RNA (siRNA) knockdown of endogenous collagen I, and cell cultivation on collagen I-coated plates versus uncoated plates. These results give important information on the role of the collagen synthesis in intestinal mucosa in the mechanism of NSAID-induced small intestinal lesions.

Crosstalk between colon cancer cells and macrophages via inflammatory mediators and CD47 promotes tumour cell migration

Tumour-associated macrophages (TAMs) of the M2 phenotype are present in the stroma of many tumours and are frequently associated with the progression of several types of cancer. We investigated the role of M2 macrophages in colon cancer progression and found that human colon cancer tissue had elevated numbers of CD68(+) (macrophage marker) cells and CD206(+) (M2 macrophage marker) cells and increased CD47 expression. To explore potential interplay between colon cancer cells and M2 macrophages, we differentiated the monocyte cell line THP-1 into M1 and M2 macrophages (CD206(high) and Th2 cytokine-secreting cells), respectively. M2 macrophages migrated faster than M1 macrophages towards SW480-conditioned medium. Similarly, M2 macrophage-conditioned medium induced SW480 cell migration and CD47 expression. Factors released by macrophages were involved in this induction. In addition, SW480 cells migrated faster when co-cultured with M2 macrophages. Inhibition of CD47 with blocking antibodies or siRNA significantly reduced the migration of SW480 cells in the presence of M2 macrophages. This effect was further decreased via blocking antibodies against the CD47 ligand signal-regulatory protein α (SIRPα). Additionally, cancer cells also secreted significant levels of IL-10, thereby promoting M2 macrophage differentiation. These findings indicate that a TAM-enriched tumour microenvironment promotes colon cancer cell migration and metastasis.

Association study identifying polymorphisms in CD47 and other extracellular matrix pathway genes as putative prognostic markers for colorectal cancer

PURPOSE

We identified recently the extracellular matrix (ECM) receptor interaction pathway as a consistently overrepresented category among gene expression profiling studies on colorectal cancer (CRC) prognosis.

METHODS

Putative regulatory single nucleotide polymorphisms (SNPs) in genes from the ECM pathway were genotyped in 613 CRC patients from Northern Germany (PopGen cohort) and tested for association with disease progression and survival.

RESULTS

The eSNP (SNP associated with expression) rs12695175 in CD47 associated with CRC specific survival (HR = 2.18, 95 % CI 1.10-4.33, CC versus AA) and with overall survival (HR = 1.99, 95 % CI 1.04-3.81, CC versus AA). This association remained significant after adjustment for age at diagnosis, tumour stage (T) and lymph node status (N). Three polymorphisms in CD47 were associated with distant metastasis in a dominant model: rs9879947 and rs3206652 in the 3'-UTR (OR = 1.64, 95 % CI 1.01-2.64 and OR = 1.88, 95 % CI 1.27-2.80, respectively) and the eSNP rs3804639 (OR = 1.73, 95 % CI 1.17-2.57).

CONCLUSIONS

The novel associations of eSNPs in CD47 with worse survival and distant metastasis should be confirmed by additional studies, since increased expression of this gene has recently been shown to be an indicator of poor prognosis in cancer patients.

CD47: A Cell Surface Glycoprotein Which Regulates Multiple Functions of Hematopoietic Cells in Health and Disease

Interactions between cells and their surroundings are important for proper function and homeostasis in a multicellular organism. These interactions can either be established between the cells and molecules in their extracellular milieu, but also involve interactions between cells. In all these situations, proteins in the plasma membranes are critically involved to relay information obtained from the exterior of the cell. The cell surface glycoprotein CD47 (integrin-associated protein (IAP)) was first identified as an important regulator of integrin function, but later also was shown to function in ways that do not necessarily involve integrins. Ligation of CD47 can induce intracellular signaling resulting in cell activation or cell death depending on the exact context. By binding to another cell surface glycoprotein, signal regulatory protein alpha (SIRPα), CD47 can regulate the function of cells in the monocyte/macrophage lineage. In this spotlight paper, several functions of CD47 will be reviewed, although some functions may be more briefly mentioned. Focus will be on the ways CD47 regulates hematopoietic cells and functions such as CD47 signaling, induction of apoptosis, and regulation of phagocytosis or cell-cell fusion.

Activation of CD47 receptors causes proliferation of human astrocytoma but not normal astrocytes via an Akt-dependent pathway

CD47 is a membrane receptor that plays pivotal roles in many pathophysiological processes, including infection, inflammation, cell spreading, proliferation, and apoptosis. We show that activation of CD47 increases proliferation of human U87 and U373 astrocytoma cells but not normal astrocytes. CD47 function-blocking antibodies inhibit proliferation of untreated U87 and U373 cells but not normal astrocytes, suggesting that CD47 may be constitutively activated in astrocytoma. CD47 expression levels were similar in our three cell types. CD47 couples to G-proteins in astrocytes and astrocytoma and especially to the Gβγ dimer. Downstream signaling following CD47 activation involves Gβγ dimer-dependent activation of the PI3K/Akt pathway in astrocytoma cells but not in normal astrocytes. This pathway is known to be deregulated in astrocytoma, leading to cell proliferation and enhanced survival signals. Putative PLIC-1 interaction with CD47 in astrocytoma cells but not astrocytes may contribute to the proliferative effect observed upon activation of CD47. Our data indicate that CD47 receptors have a stimulatory role in cell proliferation and demonstrate for the first time that CD47 signals via the PI3K/Akt pathway in cancerous cells but not normal cells.

Silencing of microRNA families by seed-targeting tiny LNAs

The challenge of understanding the widespread biological roles of animal microRNAs (miRNAs) has prompted the development of genetic and functional genomics technologies for miRNA loss-of-function studies. However, tools for exploring the functions of entire miRNA families are still limited. We developed a method that enables antagonism of miRNA function using seed-targeting 8-mer locked nucleic acid (LNA) oligonucleotides, termed tiny LNAs. Transfection of tiny LNAs into cells resulted in simultaneous inhibition of miRNAs within families sharing the same seed with concomitant upregulation of direct targets. In addition, systemically delivered, unconjugated tiny LNAs showed uptake in many normal tissues and in breast tumors in mice, coinciding with long-term miRNA silencing. Transcriptional and proteomic profiling suggested that tiny LNAs have negligible off-target effects, not significantly altering the output from mRNAs with perfect tiny LNA complementary sites. Considered together, these data support the utility of tiny LNAs in elucidating the functions of miRNA families in vivo.

Prolactin receptor-integrin cross-talk mediated by SIRPα in breast cancer cells

The hormone prolactin (PRL) contributes to the pathogenesis of breast cancer in part through its activation of Janus-activated kinase 2 (Jak2)/signal transducer and activator of transcription 5 (Stat5), a PRL receptor (PRLr)-associated pathway dependent on cross-talk signaling from integrins. It remains unclear, however, how this cross-talk is mediated. Following PRL stimulation, we show that a complex between the transmembrane glycoprotein signal regulatory protein-α (SIRPα) and the PRLr, β(1) integrin, and Jak2 in estrogen receptor-positive (ER(+)) and ER(-) breast cancer cells is formed. Overexpression of SIRPα in the absence of collagen 1 significantly decreased PRL-induced gene expression, phosphorylation of PRLr-associated signaling proteins, and PRL-stimulated proliferation and soft agar colony formation. In contrast, overexpression of SIRPα in the presence of collagen 1 increased PRL-induced gene expression; phosphorylation of Jak2, Stat5, and Erk; and PRL-stimulated cell growth. Interestingly, overexpression of a tyrosine-deficient SIRPα (SIRPα-4YF) prevented the signaling and phenotypic effects mediated by wild-type SIRPα. Furthermore, overexpression of a phosphatase-defective mutant of Shp-2 or pharmacologic inhibition of Shp-2 produced effects comparable with that of SIRPα-4YF. However, the tyrosine phosphorylation of SIRPα was unaffected in the presence or absence of collagen 1. These data suggest that SIRPα modulates PRLr-associated signaling as a function of integrin occupancy predominantly through the alteration of Shp-2 activity. This PRLr-SIRPα-integrin complex may therefore provide a basis for integrin-PRLr cross-talk and contribute to the biology of breast cancer.

Mechanism of synergy between T cell signals and C8-substituted guanine nucleosides in humoral immunity: B lymphotropic cytokines induce responsiveness to 8-mercaptoguanosine

B lymphocytes require a source of T cell-like help to produce antibody to T cell-dependent antigens. T cell-derived lymphokines and C8-substituted guanine ribonucleosides (such as 8-mercaptoguanosine; 8MGuo) are effective sources of such T cell-like help. Addition of T cell-derived lymphokines to antigen-activated B cells together with 8MGuo results in synergistic B cell differentiation, amplifying the sum of the individual responses twofold to four-fold. Lymphokine activity is required at initiation of culture for optimal synergy with 8MGuo, whereas the nucleoside can be added up to 48 hr after the lymphokines with full synergy. 8MGuo provides a perceived T cell-like differentiation signal to B cells from immunodeficient xid mice, thereby distinguishing a subset of Lyb-5- nucleoside-responsive B cells from those activated by soluble anti-mu followed by B cell stimulatory factor-1, interleukin 1, and B cell differentiation factors, which are Lyb-5+. Moreover, at least a subset of the B cells recruited by the synergistic interaction of lymphokines and nucleoside is distinct from that responsive to 8MGuo + antigen, insofar as Sephadex G-10 nonadherent xid B cells fail to respond to either 8MGuo or lymphokines alone, but do respond to the combination. A distinct subpopulation can also be demonstrated among normal B cells by limiting dilution analysis in which the precursor frequency of antigen-reactive B cells in the presence of lymphokines or nucleoside alone increases substantially when both agents are present together. In concert with the kinetic data, these observations suggest that synergy derives at least in part from the ability of lymphokines to induce one or more elements the absence of which limits the capacity of a distinct B cell subpopulation to respond to 8MGuo.