p73, the “Assistant” Guardian of the Genome?

Dichotomous transactivation domains contribute to growth inhibitory and promotion functions of TAp73

The transcription factor p73, a member of the p53 tumor-suppressor family, regulates cell death and also supports tumorigenesis, although the mechanistic basis for the dichotomous functions is poorly understood. We report here the identification of an alternate transactivation domain (TAD) located at the extreme carboxyl (C) terminus of TAp73β, a commonly expressed p73 isoform. Mutational disruption of this TAD significantly reduced TAp73β's transactivation activity, to a level observed when the amino (N)-TAD that is similar to p53's TAD, is mutated. Mutation of both TADs almost completely abolished TAp73β's transactivation activity. Expression profiling highlighted a unique set of targets involved in extracellular matrix-receptor interaction and focal adhesion regulated by the C-TAD, resulting in FAK phosphorylation, distinct from the N-TAD targets that are common to p53 and are involved in growth inhibition. Interestingly, the C-TAD targets are also regulated by the oncogenic, amino-terminal-deficient DNp73β isoform. Consistently, mutation of C-TAD reduces cellular migration and proliferation. Mechanistically, selective binding of TAp73β to DNAJA1 is required for the transactivation of C-TAD target genes, and silencing DNAJA1 expression abrogated all C-TAD-mediated effects. Taken together, our results provide a mechanistic basis for the dichotomous functions of TAp73 in the regulation of cellular growth through its distinct TADs.

Exposure of the cytoplasm to low-dose X-rays modifies ataxia telangiectasia mutated-mediated DNA damage responses

We recently showed that when a low X-ray dose is used, cell death is enhanced in nucleus-irradiated compared with whole-cell-irradiated cells; however, the role of the cytoplasm remains unclear. Here, we show changes in the DNA damage responses with or without X-ray microbeam irradiation of the cytoplasm. Phosphorylated histone H2AX foci, a surrogate marker for DNA double-strand breaks, in V79 and WI-38 cells are not observed in nucleus irradiations at ≤ 2 Gy, whereas they are observed in whole-cell irradiations. Addition of an ataxia telangiectasia mutated (ATM) kinase inhibitor to whole-cell irradiations suppresses foci formation at ≤ 2 Gy. ABL1 and p73 expression is upregulated following nucleus irradiation, suggesting the induction of p73-dependent cell death. Furthermore, CDKN1A (p21) is upregulated following whole-cell irradiation, indicating the induction of cell cycle arrest. These data reveal that cytoplasmic radioresponses modify ATM-mediated DNA damage responses and determine the fate of cells irradiated at low doses.

The DNA damage-induced cell death response: a roadmap to kill cancer cells

Upon massive DNA damage cells fail to undergo productive DNA repair and trigger the cell death response. Resistance to cell death is linked to cellular transformation and carcinogenesis as well as radio- and chemoresistance, making the underlying signaling pathways a promising target for therapeutic intervention. Diverse DNA damage-induced cell death pathways are operative in mammalian cells and finally culminate in the induction of programmed cell death via activation of apoptosis or necroptosis. These signaling routes affect nuclear, mitochondria- and plasma membrane-associated key molecules to activate the apoptotic or necroptotic response. In this review, we highlight the main signaling pathways, molecular players and mechanisms guiding the DNA damage-induced cell death response.

The Dual Estrogen Receptor α Inhibitory Effects of the Tissue-Selective Estrogen Complex for Endometrial and Breast Safety

The conjugated estrogen /: bazedoxifene tissue-selective estrogen complex (TSEC) is designed to minimize the undesirable effects of estrogen in the uterus and breast tissues and to allow the beneficial effects of estrogen in other estrogen-target tissues, such as the bone and brain. However, the molecular mechanism underlying endometrial and breast safety during TSEC use is not fully understood. Estrogen receptor α (ERα)-estrogen response element (ERE)-DNA pull-down assays using HeLa nuclear extracts followed by mass spectrometry-immunoblotting analyses revealed that, upon TSEC treatment, ERα interacted with transcriptional repressors rather than coactivators. Therefore, the TSEC-mediated recruitment of transcriptional repressors suppresses ERα-mediated transcription in the breast and uterus. In addition, TSEC treatment also degraded ERα protein in uterine tissue and breast cancer cells, but not in bone cells. Interestingly, ERα-ERE-DNA pull-down assays also revealed that, upon TSEC treatment, ERα interacted with the F-box protein 45 (FBXO45) E3 ubiquitin ligase. The loss-of- and gain-of-FBXO45 function analyses indicated that FBXO45 is involved in TSEC-mediated degradation of the ERα protein in endometrial and breast cells. In preclinical studies, these synergistic effects of TSEC on ERα inhibition also suppressed the estrogen-dependent progression of endometriosis. Therefore, the endometrial and breast safety effects of TSEC are associated with synergy between the selective recruitment of transcriptional repressors to ERα and FBXO45-mediated degradation of the ERα protein.

Functions, divergence and clinical value of TAp73 isoforms in cancer

The p73 gene encodes the tumour suppressive full-length TAp73 and N-terminal-truncated DNp73 isoforms that act as dominant negative inhibitors of TAp73. The overall effect of p73 in oncogenesis is thought to depend on the TAp73 to DNp73 isoforms' ratio. TAp73 isoforms include a number of C-terminal variants as a result of alternative splicing in 3'-end. TAp73 isoforms protect cells from oncogenic alterations in a multifaceted way since they are implicated in the suppression of all demonstrated hallmarks and enabling characteristics of cancer. Their best established role is in apoptosis, a process which seems to be differently affected by each TAp73 C-terminal variant. Based on previous findings and our thorough bioinformatics analysis, we highlight that TAp73 variants are functionally non-equivalent, since they present major differences in their transactivation efficiencies, protein interactions, response to DNA damage and apoptotic effects that are attributable to the primary structure of their C terminus. In this review, we summarise these differences and we unveil the link between crucial C-terminal motifs/residues and the oncosuppressive potential of TAp73 isoforms, emphasising on the importance of considering C terminus during the development of p73-based anticancer biologics.

Graft-versus-host disease-related cytokine-driven apoptosis depends on p73 in cytokeratin 15-positive target cells

Acute graft-versus-host disease (GVHD), a major complication of allogeneic stem cell transplantation, involves cytotoxic soluble and cellular effectors that selectively induce apoptosis in normally apoptosis-resistant, cytokeratin 15 (K15)-expressing epithelial stem cells residing at the tips of rete ridges of human epidermis and in analogous rete-like prominences (RLPs) of murine dorsal lingual epithelium. The mechanisms whereby epithelial stem cells are rendered vulnerable to apoptosis during allostimulation are unknown. We hypothesized that GVHD-induced target cell injury may be related to pathways involving the p53 family that are constitutively expressed by epithelial stem cells and designed to trigger physiological apoptosis as a result of environmental danger signals. Among the p53 family members, we found that p73 protein and mRNA were preferentially expressed in K15(+) RLPs of murine lingual squamous epithelium. On in vitro exposure to recombinant TNF-α and IL-1 in an organ culture model previously shown to replicate early GVHD-like target cell injury, apoptosis was selectively induced in K15(+) stem cell regions and was associated with induction of phosphorylated p73, a marker for p73 activation, and apoptosis was abrogated in target tissue obtained from p73-deficient (p73(-/-)) mice. Evaluation of early in vivo lesions in experimental murine GVHD disclosed identical patterns of phosphorylated p73 expression that coincided with the onset of effector T cell infiltration and target cell apoptosis within K15(+) RLPs. This study is the first to suggest that paradoxical apoptosis in GVHD of physiologically protected K15(+) epithelial stem cells is explainable, at least in part, by cytokine-induced activation of suicide pathways designed to eliminate stem cells after exposure to deleterious factors perceived to be harmful to the host.

Tumor regression following intravenous administration of a tumor-targeted p73 gene delivery system

The potential of gene therapy to treat cancer is hampered by the lack of safe and efficacious gene delivery systems able to selectively deliver therapeutic genes to tumors by intravenous administration. With the long-term aim of developing an efficacious cancer-targeted gene medicine, we demonstrated that transferrin-bearing polypropylenimine dendrimer complexed to a plasmid DNA encoding p73 led to an enhanced anti-proliferative activity in vitro, by up to 120-fold in A431 compared to the unmodified dendriplex. In vivo, the intravenous administration of this p73-encoding dendriplex resulted in a rapid and sustained inhibition of tumor growth over one month, with complete tumor suppression for 10% of A431 and B16-F10 tumors and long-term survival of the animals. The treatment was well tolerated by the animals, with no apparent signs of toxicity. These results suggest that the p73-encoding tumor-targeted polypropylenimine dendrimer should be further explored as a therapeutic strategy for cancer therapy.

The emerging role of matrix metalloproteases of the ADAM family in male germ cell apoptosis

Constitutive germ cell apoptosis during mammalian spermatogenesis is a key process for controlling sperm output and to eliminate damaged or unwanted cells. An increase or decrease in the apoptosis rate has deleterious consequences and leads to low sperm production. Apoptosis in spermatogenesis has been widely studied, but the mechanism by which it is induced under physiological or pathological conditions has not been clarified. We have recently identified the metalloprotease ADAM17 (TACE) as a putative physiological inducer of germ cell apoptosis. The mechanisms involved in regulating the shedding of the ADAM17 extracellular domain are still far from being understood, although they are important in order to understand cell-cell communications. Here, we review the available data regarding apoptosis during mammalian spermatogenesis and the localization of ADAM proteins in the male reproductive tract. We propose an integrative working model where ADAM17, p38 MAPK, protein kinase C (PKC) and the tyrosine kinase c-Abl participate in the physiological signalling cascade inducing apoptosis in germ cells. In our model, we also propose a role for the Sertoli cell in regulating the Fas/FasL system in order to induce the extrinsic pathway of apoptosis in germ cells. This working model could be applied to further understand constitutive apoptosis in spermatogenesis and in pathological conditions (e.g., varicocele) or following environmental toxicants exposure (e.g., genotoxicity or xenoestrogens).

Genetic variants of the p53 and p73 genes jointly increase risk of second primary malignancies in patients after index squamous cell carcinoma of the head and neck

BACKGROUND

Because of the structural and biochemical similarities between the antitumor p53 and p73 proteins, the authors hypothesized that individuals who carry high-risk genotypes of p53 codon 72 and p73 G4C14-to-A4T14 polymorphisms have a higher risk of developing second primary malignancy (SPM) after index squamous cell carcinoma of the head and neck (SCCHN).

METHODS

A cohort of 1269 patients with index cases of SCCHN was recruited between May 1995 and January 2007 at The University of Texas MD Anderson Cancer Center and followed for SPM development. Patients were genotyped for p53 codon 72 and p73 G4C14-to-A4T14 polymorphisms. A log-rank test and Cox proportional hazard models were used to compare SPM-free survival and SPM risk among different risk groups with the combined risk genotypes of the 2 polymorphisms.

RESULTS

The data demonstrated that patients with p53 WP + PP and p73 GC/GC genotypes had a worse SPM-free survival and an increased SPM risk compared with the corresponding p53 WW and p73 GC/AT + AT/AT genotypes. After combining the 2 polymorphisms, a borderline significantly or significantly reduced SPM-free survival and increased SPM risk were observed in the medium-risk group (p53 WW and p73 GC/GC or p53 P carriers and p73 AT carriers) and high-risk group (p53 P carriers and p73 GC/GC) compared with low-risk group (p53 WW and p73 AT carriers), respectively.

CONCLUSIONS

The results suggest an increased risk of SPM after index SCCHN with both p53 and p73 polymorphisms individually and in combination.

The F-box protein FBXO45 promotes the proteasome-dependent degradation of p73

The transcription factor p73, a member of the p53 family, mediates cell-cycle arrest and apoptosis in response to DNA damage-induced cellular stress, acting thus as a proapoptotic gene. Similar to p53, p73 activity is regulated by post-translational modification, including phosphorylation, acetylation and ubiquitylation. In C. elegans, the F-box protein FSN-1 controls germline apoptosis by regulating CEP-1, the single ancestral p53 family member. Here we report that FBXO45, the human ortholog of FSN-1, binds specifically to p73 triggering its proteasome-dependent degradation. Importantly, SCF(FBXO45) ubiquitylates p73 both in vivo and in vitro. Moreover, siRNA-mediated depletion of FBXO45 stabilizes p73 and concomitantly induces cell death in a p53-independent manner. All together, these results show that the orphan F-box protein FBXO45 regulates the stability of p73, highlighting a conserved pathway evolved from nematode to human by which the p53 members are regulated by an SCF-dependent mechanism.

TAp73 regulates the spindle assembly checkpoint by modulating BubR1 activity

The role of various p73 isoforms in tumorigenesis has been controversial. However, as we have recently shown, the generation of TAp73-deficient (TAp73(-/-)) mice reveals that TAp73 isoforms exert tumor-suppressive functions, indicating an emerging role for Trp-73 in the maintenance of genomic stability. Unlike mice lacking all p73 isoforms, TAp73(-/-) mice show a high incidence of spontaneous tumors. Moreover, TAp73(-/-) mice are infertile and produce oocytes exhibiting spindle abnormalities. These data suggest a link between TAp73 activities and the common molecular machinery underlying meiosis and mitosis. Previous studies have indicated that the spindle assembly checkpoint (SAC) complex, whose activation leads to mitotic arrest, also regulates meiosis. In this study, we demonstrate in murine and human cells that TAp73 is able to interact directly with several partners of the SAC complex (Bub1, Bub3, and BubR1). We also show that TAp73 is involved in SAC protein localization and activities. Moreover, we show that decreased TAp73 expression correlates with increases of SAC protein expression in patients with lung cancer. Our results establish TAp73 as a regulator of SAC responses and indicate that TAp73 loss can lead to mitotic arrest defects. Our data suggest that SAC impairment in the absence of functional TAp73 could explain the genomic instability and increased aneuploidy observed in TAp73-deficient cells.

Specific activation of microRNA106b enables the p73 apoptotic response in chronic lymphocytic leukemia by targeting the ubiquitin ligase Itch for degradation

Chronic lymphocytic leukemia (CLL) is characterized by cells that exhibit dysfunctional apoptosis. Here, we show that deacetylase inhibition led to the E2F1- and myc-mediated transcriptional activation of the microRNA miR106b in primary CLL cells. Induction of miR106b was associated with a down-regulation in the levels of the E3-ubiquitin ligase Itch. Decreases in Itch protein levels were associated with a reciprocal accumulation of its proapoptotic substrate, TAp73 (p73), and induction of p53 up-regulated modulator of apoptosis (PUMA) mRNA and protein. This event was accompanied by mitochondrial dysfunction, processing of caspase-9, and apoptosis of CLL cells. Ectopic expression of miR106b in CLL cells demonstrated that Itch was a direct target of miR106b such that miR106b-induced decreases in Itch resulted in an accumulation of p73. Thus, our results identify a novel regulatory mechanism wherein microRNA regulate cell survival by mediating the posttranscriptional down-regulation of an ubiquitin ligase, leading to the induction of a proapoptotic regulator in malignant cells. Silencing of miRNA expression in CLL may selectively suppress proapoptotic pathways, providing such tumors with a survival advantage. Consequently, chemotherapeutic drugs that activate miR106b could initiate a p53-independent mechanism that targets CLL cells.

Healthy individuals have Goodpasture autoantigen-reactive T cells

Autoreactive T cells in patients with Goodpasture's disease are specific for epitopes in the Goodpasture antigen (the NC1 domain of the alpha3 chain of type IV collagen) that are rapidly destroyed during antigen processing to a degree that diminishes their presentation to T cells. We hypothesized that patients' autoreactive T cells exist because antigen processing prevents presentation of the self-epitopes they recognize, circumventing specific tolerance mechanisms. We predicted that autoreactive T cells specific for these peptides should also exist in healthy individuals, albeit at low frequency and in an unprimed state. We obtained blood from healthy unrelated donors and, using a panel of 45 alpha3(IV)NC1 peptides, identified alpha3(IV)NC1-specific T cells in all donors. Thirty-six of 45 peptides elicited a proliferative T cell response from at least one subject, and 6 of the peptides evoked a response in >50% of the individuals. This consistency was not caused by selectivity of HLA class II molecules because the donors expressed a diversity of HLA antigens, but was largely a result of the substrate-specificity of the endosomal proteases Cathepsin D and E. There was a significant correlation between high susceptibility to Cathepsin D digestion and the capacity to stimulate primary T cell responses (P = 0.00006). In summary, healthy individuals have low frequencies of unstimulated alpha3(IV)NC1-reactive T cells with similar specificities to the autoreactive T cells found in patients with Goodpasture disease. In both cases, existence of the alpha3(IV)NC1-reactive T cells can be accounted for by destructive processing.

Abrogation of p53 by its antisense in MCF-7 breast carcinoma cells increases cyclin D1 via activation of Akt and promotion of cell proliferation

The p53 protein has been a subject of intense research interest since its discovery as about 50% of human cancers carry p53 mutations. Mutations in the p53 gene are the most frequent genetic lesions in breast cancers suggesting a critical role of p53 in breast cancer development, growth and chemosensitivity. This report describes the derivation and characterization of MCF-7As53, an isogenic cell line derived from MCF-7 breast carcinoma cells in which p53 was abrogated by antisense p53 cDNA. Similar to MCF-7 and simultaneously selected hygromycin resistant MCF-7H cells, MCF-7As53 cells have consistent basal epithelial phenotype, morphology, and estrogen receptor expression levels at normal growth conditions. Present work documents investigation of molecular variations, growth kinetics, and cell cycle related studies in relation to absence of wild-type p53 protein and its transactivation potential as well. Even though wild-type tumor suppressor p53 is an activator of cell growth arrest and apoptosis-mediator genes such as p21, Bax, and GADD45 in MCF-7As53 cells, no alterations in expression levels of these genes were detected. The doubling time of these cells decreased due to depletion of G0/G1 cell phase because of constitutive activation of Akt and increase in cyclin D1 protein levels. This proliferative property was abrogated by wortmannin, an inhibitor of PI3-K/Akt signaling pathway. Therefore this p53 null cell line indicates that p53 is an indispensable component of cellular signaling system which is regulated by caveolin-1 expression, involving Akt activation and increase in cyclin D1, thereby promoting proliferation of breast cancer cells.

Identification of DeltaN isoform and polyadenylation site choice variants in molluscan p63/p73-like homologues

The p53 family of transcription factors has been implicated in many vertebrate cancers. Altered p53 and p73 protein expression observed in leukemic cells of molluscs suggests that these transcription factors might be involved in invertebrate cancers as well. Here, we fully characterize the mRNA of four novel p53-like variants in the bivalve molluscs Mytilus trossulus (bay mussel) and Mytilus edulis (blue mussel). These species, widely used for environmental assessment, develop a hemic neoplasia (leukemia) that is frequently fatal. The correlation between expression of p53 and its close relative p73 and onset of molluscan leukemia was documented previously. We report the sequences of two distinct and novel p63/p73-like mRNAs, amplified by polymerase chain reaction (PCR) from both species. One of the p63/p73-like isoforms contains a 360 nt truncation in the 5' coding region. Based on this truncation and concomitant lack of a transactivation (TA) domain, we designate this variant as a DeltaNp63/p73-like isoform: the first to be reported in an invertebrate species. In mammalian species, DeltaNp73 potently inhibits the tumor-suppressive function of p73 and p53, and its overexpression serves as a robust marker for mammalian cancer. In addition, we report on the occurrence of alternate polyadenylation sites in the molluscan p63/p73: one proximal and one distal site, which differ by 1260 nt. We hypothesize that differential expression of various molluscan p63/p73-like isoforms, controlled in part by polyadenylation site choice variation, may help to interpret the apparently opposing roles of this gene in the development of cancer. Overall, this research further illustrates the utility of the molluscan model for studies involving the molecular mechanisms of oncogenesis in naturally occurring populations. The data presented here require a revisiting of hypotheses regarding evolution of the p53 gene family. Current hypotheses indicate that (1) the protostome gene family does not contain an intronic promoter for DeltaN expression and (2) p53 gene duplication did not occur in protostomes. Our characterization of DeltaN p63/73 in mussel suggests that molluscan p53 gene family members have acquired an intronic promoter or splicing mechanism, either by invention that predates the evolutionary split of deuterostomes from protostomes, or by parallel evolution. Our data also show that Mytilus p53, p63/p73, and DeltaNp63/p73 are identical in their core regions with variation limited to their C- and N-terminals, supporting the notion that alternative splicing, intronic promoter usage, and polyadenylation site choice may lead to expression of distinct isoforms originating from one common gene.

Presentation of the Goodpasture Autoantigen Requires Proteolytic Unlocking Steps That Destroy Prominent T Cell Epitopes

The most abundant autoreactive T cells in patients with Goodpasture's disease are specific for peptides in the autoantigen that have high affinity for the disease-associated HLA class II molecule, DR15. How can such T cells escape self-tolerance mechanisms? This study showed that these peptides are highly susceptible to destruction in the earliest stages of antigen processing, and some must be cleaved for antigen digestion to be possible ("unlocking"). Goodpasture autoantigen [collagen alpha3(IV)NC1; approximately 31 kD] that was incubated with B cell lysosomes was cleaved within a few minutes to form approximately 9- and approximately 22-kD fragments, then increasing quantities of smaller peptides. The processing was completely abrogated by pepstatin A, a specific inhibitor of cathepsin D/E, even though lysosomal extracts contain a rich array of proteases. Purified cathepsin D generated the same major alpha3(IV)NC1 fragments as entire lysosomes, suggesting that cathepsin D cleavages are required to initiate alpha3(IV)NC1 processing. The initial unlocking cleavages destroyed two major self-epitopes, and subsequent preferred cleavages destroyed all of the other T cell epitopes that are recognized by most patients' autoreactive T cells. The responses of T cell clones that are specific for a major disease-associated peptide to antigen-pulsed intact antigen-presenting cells were substantially enhanced by pepstatin A treatment. Therefore, cathepsin D activity significantly diminishes presentation of alpha3(IV)NC1 peptides that are recognized by patients' T cells by destroying the peptides in early processing. These observations can explain why the mature T cell repertoire includes reactivity toward these self-peptides and suggests that a key factor in disease initiation is likely to be a shift in antigen processing.

Differential Processing of Autoantigens in Lysosomes from Human Monocyte-Derived and Peripheral Blood Dendritic Cells

Dendritic cells (DC) initiate immunity and maintain tolerance. Although in vitro-generated DC, usually derived from peripheral blood monocytes (MO-DC), serve as prototype DC to analyze the biology and biochemistry of DC, phenotypically distinct primary types of DC, including CD1c-DC, are present in peripheral blood (PB-DC). The composition of lysosomal proteases in PB-DC and the way their MHC class II-associated Ag-processing machinery handles a clinically relevant Ag are unknown. We show that CD1c-DC lack significant amounts of active cathepsins (Cat) S, L, and B as well as the asparagine-specific endopeptidase, the major enzymes believed to mediate MHC class II-associated Ag processing. However, at a functional level, lysosomal extracts from CD1c-DC processed the multiple sclerosis-associated autoantigens myelin basic protein and myelin oligodendrocyte glycoprotein in vitro more effectively than MO-DC. Although processing was dominated by CatS, CatD, and asparagine-specific endopeptidase in MO-DC, it was dominated by CatG in CD1c-DC. Thus, human MO-DC and PB-DC significantly differ with respect to their repertoire of active endocytic proteases, so that both proteolytic machineries process a given autoantigen via different proteolytic pathways.

Regulation of gene-activation pathways by PIAS proteins in the immune system

The protein inhibitor of activated STAT (PIAS) family of proteins has been proposed to regulate the activity of many transcription factors, including signal transducer and activator of transcription proteins (STATs), nuclear factor-kappaB, SMA- and MAD-related proteins (SMADs), and the tumour-suppressor protein p53. PIAS proteins regulate transcription through several mechanisms, including blocking the DNA-binding activity of transcription factors, recruiting transcriptional corepressors or co-activators, and promoting protein sumoylation. Recent genetic studies support an in vivo function for PIAS proteins in the regulation of innate immune responses. In this article, we review the current understanding of the molecular basis, specificity and physiological roles of PIAS proteins in the regulation of gene-activation pathways in the immune system.

Basic Concepts of Immune Response and Defense Development

The induction of immune responses requires critical interaction between innate parts of the immune system, which respond rapidly and in a relatively nonspecific manner, and other specific parts, which recognize particular epitopes on an antigen. A critical element in this interaction is the role played by dendritic cells (DCs), which represent "professional antigen-presenting cells." DCs endocytose and process antigen to peptide presented on the cell surface in association with major histocompatibility complex (MHC) molecules. This presentation results in interaction with and stimulation of helper T (Th) lymphocytes, which recognize peptide in association with either MHC class II or cytotoxic T (Tc) lymphocytes, which recognize peptide in association with MHC class I. Stimulation of Th lymphocytes produces the growth and differentiation factors (cytokines) essential for the B lymphocytes that have responded to a more intact form of the antigen and that differentiate into antibody-producing cells. The precise interaction between the cells depends on cognate ligand-receptor recognition between the B and Th lymphocytes. DCs also play a direct role with the stimulation of the B lymphocytes. It appears that DC can deliver antigen to the B lymphocytes in a more intact form than the processed form essential for stimulating T lymphocytes, and can release cytokines that assist the differentiation of the B lymphocytes into antibody-producing cells. This close relationship among the three cell types and the cytokines that are produced ensures the precise control and regulation necessary for immune response development.