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The bursa of Fabricius, the acknowledged central humoral immune organ, plays a vital role in B lymphocyte differentiation. However, there are few reports of the molecular basis of the mechanism on immune induction and potential antitumor activity of bursal-derived peptides. In this paper, a novel bursal-derived pentapeptide-II (BPP-II, MTLTG) was isolated and exerted immunomodulatory functions on antibody responses in vitro. Gene microarray analyses demonstrated that BPP-II regulated expression of 2478 genes in a mouse-derived hybridoma cell line. Immune-related gene ontology functional procedures were employed for further functional analysis. Furthermore, the majority of BPP-II-regulated pathways were associated with immune responses and tumor processes. Moreover, BPP-II exhibited immunomodulatory effects on antigen-specific immune responses in vivo, including enhancement of avian influenza virus (H9N2 subtype)-specific antibody and cytokine production and modification of T cell immunophenotypes and lymphocyte proliferation. Finally, BPP-II triggered p53 expression and stabilization and selectively inhibited tumor cell proliferation. These data identified the multifunctional factor, BPP-II, as a novel biomaterial representing an important linking between the humoral central immune system and immune induction, including antitumor. Information generated in this study elucidates further the mechanisms involved in humoral immune system and represents the potential basis of effective immunotherapeutic strategies for treating human tumors and immune improvement.
Two separate differentiation pathways for lymphocytes are established, one for T (thymic) lymphocytes and the other for antibody-secreting B (bone marrow and bursal) lymphocytes. In mammals and human, the B cell-differentiating organ equivalent to the T cell-differentiating thymus has not been defined (1, 2). The bursa of Fabricius (BF)2 is the acknowledged central humoral immune organ (3), which is vital to B differentiation and antibody production (4). B cell is named after “bursal-derived lymphocyte.” Therefore, BF provides an invaluable model for studies on the basic immunology of mammals and human.
Various immunomodulatory peptides have been isolated from BF. Bursin, a specific molecule for the differentiation of B cells (5), selectively induced avian B cells, but not avian T cells, from their precursors in vitro (1, 2) and promoted immunoglobulin switching from IgM to IgG (6). Bursal septpeptide-II (BSP-II, TPSGLVY) induced various immune responses in vivo and regulated tumor cells proliferation (7). Bursopentin was proved to induce B lymphocyte proliferation through activating various pathways, such as MAPK and NF-κB signals (8). Also, bursal septapeptide-I and bursal pentapeptide (BPP)-I have antiproliferative effects on the tumor cells and the initiation of p53 expression, an important tumor suppressor (9, 10).
However, the molecular basis and potential mechanisms by which BF stimulates and regulates immune response are not fully understood. Therefore, it is important to study the mechanisms and cellular basis of active peptides derived from BF on basic immunology. In this paper, a novel bursal-derived immune-inducing BPP-II was isolated, and the induced downstream signaling pathways and biological consequences were investigated using gene microarrays to characterize the potential mechanisms by which BF functions in immunity and tumorigenesis. Also, BPP-II exerted significant immunomodulatory effects on both humoral and cellular-mediated immune responses. It was demonstrated that BPP-II activated the tumor suppressor p53 expression with strong antiproliferation on tumor cells, thus providing an insight into the link between the humoral central immune system and immune induction, including antitumor. These data indicated the potential basis of immune induction and immunotherapeutic strategies for the treatment of cancer and immune improvement.
BALB/c female mice (6–8 weeks old, 17–21 g) were obtained from Yang Zhou University (Yangzhou, China). All of the animal experimental procedures were performed in accordance with the institutional ethical guidelines for animal experiments. Hybridoma cells (1H5F9 strain, IgG1 κ subtype antibody) (10), were cultured with RPMI 1640 medium supplemented with 20% heat-inactivated fetal bovine serum (FBS; Invitrogen) at 37 °C with 5% CO2. Tumor cell lines MCF-7 and HeLa and normal cell lines CEF, BHK21, MDBK, and Vero were cultured with DMEM supplemented with 10% FBS at 37 °C with 5% CO2.
Bursal peptide was purified from avian BF by reversed-phase (RP) high performance liquid chromatography (HPLC), according to methods described previously (7–10) with some slight modifications. Briefly, a BF extract prepared by homogenization and centrifugation was ultrafiltered (lower than 1000 Da) for 48 h at 4 °C and filtered (0.22 μm) and analyzed using a 4.6 × 250-mm SinoChrom ODS-BP RP-HPLC affinity column (Elite) with a linear gradient of acetonitrile (2–100%) and monitored at 220 nm. The elution was collected and analyzed using matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) (Bruker). The bursal-derived peptide was synthesized with purity >97.8%.
Hybridoma cells (105 cells/ml) were prepared in 96-well plates and treated with or without BPP-II (20, 2, 0.2, and 0.02 μg/ml). After 48 h, the viability was determined with the MTT reagent (Sigma) (11, 12), and the supernatant antibody titers were determined by ELISA method (7).
Total RNA was harvested from 0.2 μg/ml BPP-II-treated hybridoma cells using TRIzol reagent (Invitrogen) according to the instructions provided by the manufacturer. RNA was amplified, labeled, and hybridized with microarrays and analyzed using the Agilent G2505B microarray scanner. The resulting data were analyzed by the Agilent GeneSpring GX software (version 11.0) system, a knowledge-based system of computer algorithms (13), and the microarray data sets were normalized in GeneSpring GX using the Agilent FE one-color scenario (mainly median normalization). Differentially expressed genes were identified through fold-change screening. GO analysis and Pathway Analysis were performed on this subset of genes.
RNA was prepared from BPP-II-treated hybridoma cell using the TRIzol reagent. The primer pairs can be found in supplemental Table S1, and regulated genes were estimated using a One Step SYBR® PrimeScript® RT-PCR kit (Takara, Shiga, Japan).
The immunomodulatory roles of BPP-II were investigated in female BALB/c mice (6–8 weeks old), as reported previously (7), in which mice were immunized intraperitoneally with a 0.2-ml inactivated avian influenza virus (AIV, H9N2 subtype) antigen containing 10, 50, and 250 μg/ml in the presence or absence of BPP-II on days 0 and 14, respectively. PBS was used as a negative control, and AIV/H9N2 vaccine served as a positive control. The sera were collected on the 14th and 28th days to detect the antigen-specific antibody responses (IgG, IgG1, and IgG2a) by ELISA method (7), respectively, and sera were collected on 7th day after the final immunization for measurement of IL-4 and IFN-γ cytokines using ELISA kits (R&D). Also, T cell immunophenotyping of spleen lymphocytes from immunized mice was performed by three-color flow cytometry analysis (BD, LSR) using mixtures of specific anti-mouse mAbs CD3, CD4, and CD8 labeled with PE, FITC or PE-Cy5. Spleen cells were isolated from immunized mice and stimulated for 48 h as described previously (7) to measure cell viability by MTT incorporation (11, 12). Additionally, the isolated splenocytes was restimulated with 10 μg/ml BPP-II or AIV antigen, and supernatant antibody and cytokine production was measured after 48 h as described previously (7).
The wild-type p53 Vero cell line was transfected with the indicated p53-Luc plasmids containing the luciferase reporter gene cloned under the control of p53-binding DNA sequences using a Lipofectamine 2000/DNA conjugate according to the manufacturer's instructions (Invitrogen). After 24 h, the transfected cells were stimulated with BPP-II (0.02–20 μg/ml) for 24 h. Also, the transfected Vero cells were preincubated for 2 h with the p53 inhibitor 20 μm α-pifithrin (14) and treated with 0.2 μg/ml BPP-II for 22 h. The p53 relative luciferase activity was assayed using a Dual Luciferase Reporter Assay system (Promega).
Nontransfected Vero cells were treated with or without BPP-II (0.02–20 μg/ml) for 24 h. Cells were treated with 1 μg/ml doxorubicin (Sigma) which has been reported to induce p53 expression (15), in parallel as a positive control. The cells were lysed with cell culture lysis reagent (Promega), and protein samples were collected. Western blotting was performed as described previously (16) using mouse anti-human p53 (DO-1; Santa Cruz Biotechnology), mouse anti-human β-actin (AC-15; Sigma), and rabbit anti-human Bax (N-20; Santa Cruz Biotechnology), to detect protein expression of p53 and Bax, in which Bax is a key component of apoptotic cascades (17). Vero cells were treated with or without 2 μg/ml BPP-II for 16 h and subsequently exposed to 100 μg/ml cycloheximide (Sigma), and incubated for the indicated periods at 37 °C.
Tumor cells MCF-7 and HeLa were added to 96-well flat-bottomed microtiter plates (100 μl/well of 2 × 105 cells/ml) and were stimulated with or without BPP-II (0.04–50 μg/ml). Also, four normal cell lines (BHK21, CEF, MDBK, and Vero) were treated with or without BPP-II at concentrations that ranged from 0.4 to 50 μg/ml for 48 h. The cell proliferation was measured, using a standard MTT-based method (11, 12), and data were statistically analyzed with SPSS software.
Results were expressed as means ± S.D. The statistical significance of the observed differences was analyzed by t tests or one-way analysis of variance (ANOVA) followed by the Student-Newman-Keuls test. A p value <0.05 was considered to be a significance level.
A relatively abundant novel polypeptide was isolated by RP-HPLC from avian BF with an elution time of 21.40 min (Fig. 1A). Characterization of the peptide termed BPP-II with a sequence of MTLTG was identified by MALDI-TOF analysis (Fig. 1B). The chemical formula (Fig. 1B) and titration curve (Fig. 1C) were analyzed by DNASTAR. The isoelectric point of BPP-II is 5.55 with a negative charge of 0.09 (Fig. 1C).
Alignment of the amino acid sequence to BPP-II was carried out using all five possible linearized peptides as query sequences in the National Center for Biotechnology Information nonredundant and Expressed Sequence Tags data bases. Screening of the identical polypeptide sequences identified revealed homology with bactericidal/permeability-increasing protein-like 3 (BPIL3) and sterile α motif domain containing 8 (SAMD8) in Gallus gallus (supplemental Table S2). Furthermore, high level homology was identified (one amino acid discrepancy) with proteins including MHC class I antigen and several factors in ubiquitin-conjugating enzyme E2 variants (supplemental Table S2). The synthetic BPP-II was identical to natural BPP-II as determined by RP-HPLC (Fig. 1D). Therefore, these data confirmed the structure of natural BPP-II.
Hybridoma cell antibody and proliferative responses were determined following BPP-II treatment for 48 h. Increased antibody responses were observed after treatment with a range of BPP-II concentrations (0.02, 0.2, 2, and 20 μg/ml) (Fig. 2A). Enhanced cell viability was detected only at 0.02 μg/ml BPP-II (Fig. 2B). These results demonstrated the regulation of BPP-II on antibody responses in vitro.
cDNA microarray systems have been used widely to investigate gene expression patterns and functional classification and to identify the specific suppression of the humoral immune response in insects (18). In this paper, to elucidate the inducing effect of BPP-II on immune cells, hybridoma cells treated for 4 h with 0.2 μg/ml BPP-II were compared with PBS-treated control hybridoma cells by mouse cDNA microarray analysis. BPP-II-associated changes (≥1.5-fold) in gene expression (1770 genes up-regulated; 708 genes down-regulated) are summarized in supplemental Table S3. That multiple transcriptional changes associated with BPP-II treatment suggested that bioactive BPP-II might be involved in various cellular processes. Selective analyses of genes associated with immune-related functional processes and signaling pathways are summarized in supplemental Table S4.
Genes differentially expressed between the BPP-II-treated and PBS control groups were categorized according to Gene Ontology (GO) (19). Given the fact that BPP-II has a broad range of effects on immune response, it is conceivable that there might be multiple gene transcriptional changes caused by BPP-II. FlyBase orthologs of mouse genes from BPP-II-induced hybridoma that were differentially expressed were used for GO enrichment analyses of immune related genes (Fig. 3 and supplemental Table S5). Analysis revealed 63 up-regulated genes and 18 down-regulated genes involved in immune system processes after BPP-II treatment (Fig. 3 and supplemental Table S5). It was identified that various genes are involved in T cell-related cellular processes, including T cell activation, differentiation, and proliferation. Also, the induced gene Fyn is involved in both T cell receptor binding and T cell receptor complex, respectively.
Gene expression related to B cell-mediated immunity was induced by BPP-II, including STAT6, Fas, protein-tyrosine phosphatase, receptor type, C (Ptprc), Ptpn6, protein kinase Cδ (Prkcd), interferon regulatory factor 7 (Irf7), and CD55 antigen (Cd55) (Fig. 3 and supplemental Table S5). However, Ms4a2 was down-regulated, and the receptor for Fc fragment of IgE, high affinity I, α polypeptide (Fcer1a), was induced by BPP-II treatment. Genes related to cytokine production (GO 0001816) and negative regulation (GO 0001818) were overrepresented in the ontology of biological processes (Fig. 3 and supplemental Table S5). These results suggested that BPP-II might participant in B cell-mediated immune responses, cytokine production, and related signals.
Confirmation of the statistically significant cell processes activated by BPP-II treatment was obtained by classification of genes with significantly modified expression based on known KEGG pathway information (supplemental Table S6). In this paper, pathways with p values <0.05 were considered statistically differentially regulated. The pathway analysis revealed that 16 pathways were regulated following BPP-II treatments (supplemental Table S6), the majority of which were associated with immune response and tumor processes, suggesting that BPP-II might trigger various immune-related cellular signals, resulting in various biological consequences.
To confirm biological validation of microarray results, the expressions of 11 immune-related genes after BPP-II treatment were determined using quantitative RT-PCR (Fig. 4). Compared with the PBS-treated control, BPP-II-treated hybridoma cells expressed significantly lower levels of MS4A2, CD86, CDKN2A, and Fgf8 genes, and significantly higher levels of CD80, Fas, CBLC, Csf1r, and CD3D genes, within 4 h of treatment (p < 0.05), as predicted in the microarray analysis, with the exception of FGF21 and Fyn.
Serum levels of IgG antibodies and IgG1 and IgG2a subtypes were measured by ELISA to evaluate the effects of BPP-II on humoral and cellular immune responses to immunization, respectively. Increased IgG antibody levels were observed in mice immunized with HIV and BPP-II, compared with AIV immunization alone, and IgG1 antibody was the predominant antibody subtype (Fig. 5A). Furthermore, inactivated AIV and BPP-II strongly enhanced Th2 (IL-4) and Th1 (IFN-γ) type cytokine productions in a dose-dependent manner (Fig. 5B). These data indicated that BPP-II immunization tends to induce a balance of Th1 and Th2 type immune responses in mice.
Splenocytes from immunized mice were isolated and restimulated with 10 μg/ml BPP-II or AIV antigen. It was observed that production of antibodies and the levels of cytokines IL-4 and IFN-γ production were higher in the supernatant of BPP-II-treated splenocytes from immunized mice than that of PBS control, respectively (supplemental Fig. S1). Also, the significantly increased productions of IL-4 and IFN-γ were observed in spleen cell cultures from immunized mice after AIV stimulation (supplemental Fig. S1B) compared with PBS control. These results suggested that BPP-II exerts the immunomodulatory functions in vitro.
To assess T cell immunophenotyping and lymphocyte proliferation response through BPP-II treatment, spleen lymphocytes were prepared at the 7th day after the second immunization. T cell populations were increased in mice that received AIV antigen and BPP-II, compared with those of mice receiving AIV antigen alone (Fig. 5C). Splenic lymphocyte proliferation, measured using the MTT method (11, 12), was significantly greater in mice immunized with AIV antigen and 50 μg/ml BPP-II compared with AIV antigen alone (Fig. 5D).
It was observed that the expressions of genes involved in p53 signal pathway was regulated at the transcriptional level after BPP-II treatment (supplemental Table S6). To determine the effect of BPP-II on p53, Vero cells were transfected with the p53 luciferase (p53-Luc) reporter plasmid encoding 14 tandem repeats of the p53 consensus binding sites (Stratagene; La Jolla, CA), and luciferase activity was assayed with or without BPP-II treatment. BPP-II exhibited strong dose-dependent activation of p53 luciferase activity (Fig. 6A), which was inhibited by 83.3% after α-pifithrin treatment (Fig. 6B). Western blotting analysis revealed dose-dependent up-regulation of p53 and target Bax protein expression by BPP-II treatment (Fig. 6, C and D). However, the expressions of p53 mRNA in Vero cells were not affected by BPP-II treatment (data not shown). These data suggested that BPP-II induces p53 activity and the expression of p53 protein.
To test whether accumulation of p53 was dependent on the regulation of protein stability, BPP-II- and PBS-treated Vero cells were treated with 100 μg/ml cycloheximide, an inhibitor of protein biosynthesis in eukaryotic cells. The p53 protein decay was monitored by Western blot analysis (Fig. 6E). The half-life of p53 in PBS-treated Vero cells (~0.5 h) was noticeably shorter than in BPP-II-treated Vero cells (Fig. 6E). In contrast, there was no significant difference in the stability of β-actin between the PBS- and BPP-II-treated cells. These results indicated that p53 protein was stabilized in BPP-II-treated Vero cells.
It was observed that BPP-II exerted a dual dose-dependent effect on tumor cell proliferation measured by MTT assay (11, 12). MCF-7 cell proliferation was inhibited by 18.8, 11.1, and 3.6% following BPP-II treatment at 50, 20, and 5 μg/ml, respectively, whereas proliferation was increased by 1.87, 21.6, and 18.44% following BPP-II treatment at 1, 0.2, and 0.04 μg/ml, respectively (Fig. 7A). HeLa cell proliferation following BPP-II treatment at 50, 20, and 5 μg/ml was lower than that of the control groups by 29.0, 17.9, and 9.6%, respectively (Fig. 7A), whereas proliferation was enhanced by 13.0, 42.57, and 31.86% following BPP-II treatment of 1, 0.2, and 0.04 μg/ml, respectively.
To identify potential selective antiproliferation on tumor cell of high BPP-II concentrations, the effects BPP-II treatment on proliferation of four normal cell lines (CEF, BHK21, Vero, and MDBK) were investigated. The results showed that 5 μg/ml BPP-II significantly enhanced CEF cell proliferation by 26.34% (p < 0.05), and BPP-II significantly induced MDBK cell proliferation by 26.22, 54.93, 69.69, 88.32, and 45.03% in a dose-dependent manner over the range from 0.4 to 50 μg/ml (5 and 10 μg/ml, p < 0.001; 2 and 50 μg/ml, p < 0.01; 0.4 μg/ml, p < 0.05), compared with that of control without BPP-II treatment (Fig. 7B). However, there no significant inhibition was observed in cell proliferation of BPP-II-treated BHK21 or Vero cells at reachable concentrations (Fig. 7B). These results suggested that no inhibition of proliferation of the four normal cell lines was observed following BPP-II treatment at experimentally achievable concentrations.
Here, we report identification of inducing immune response and characterization of the profile of gene expression regulated after BPP-II treatment, a new isolated biological peptide from the humoral central immune organ, BF. These will only be selectively discussed.
In this study, a new bursal pentapeptide (BPP-II), MTLTG is the first reported (Fig. 1). BPP-II was found to be homology with various proteins in G. gallus (supplemental Table S2), which are related to the innate immune system, including defense against bacterial, viral, and fungal infections (20), ubiquitination process (21), and antitumor (22, 23). These proteins are conserved in various species, including chicken, human, chimpanzee, dog, mouse, and rat. Identification of functional homology proteins will provide elucidation of the function of BF in experimental study and clinical application. However, the potential inducing functions of BPP-II in various immune responses remain to be defined.
For sharing characteristic of B lymphocyte-secreting antibody, hybridoma cell was used as immunocyte model to study further the potential molecular basis of the regulation of BPP-II on immune responses (supplemental Table S3). The results of microarray analysis showed BPP-II regulated numerous genes involved in various immune signaling (supplemental Table S6), which provided an opportunity to survey the potential mechanism of BPP-II on immune responses. The key question is: could all of the involved genes of activated signals in concert cause the observed regulation in immune-related signaling? To consider how BPP-II signaling converges on immune signaling, we will briefly consider the roles of genes and their possible roles in activation of immune signaling.
Microarray analysis demonstrated that BPP-II regulated expression of various genes involved in T cell activation (Fig. 3 and supplemental Table S5). Furthermore, immunization experiments revealed that BPP-II altered the T cell phenotype of immune responses (Fig. 5C), which suggested involvement of BPP-II in T cell signaling. Key steps during T cell activation are the activation of Ras and Rho family GTPase, which are also important targets for the products of PI3K. Pip5k1c regulates the adhesion through facilitating RhoA GTPase and integrin activation by a Rho guanine nucleotide exchange factor 12 (Arhgef12) (24, 25), which was up-regulated after BPP-II treatment (supplemental Table S4). Furthermore, Pip5k1c(r) contributes to the critical second messenger inositol trisphosphate and diacylglycerol (26). The up-regulation of CACNB4 expression and down-regulation of CACNA1I expression following BPP-II treatment (supplemental Table S4) suggest that BPP-II affects the second messenger Ca2+, leading to immune responses on effector cells (27, 28). Optimal T cell activation requires Ca2+/calcineurin-NFAT signaling in concert with Ras/MAPK activation. These results indicated that BPP-II might participant the second messenger inositol trisphosphate pathway through voltage-dependent calcium channel to alter T cell activation.
JAK-STAT pathway plays vital roles on T cell activation, cell cycle, proliferation, and differentiation of effectors cells. Modulation of various genes encoding interleukins and interleukin receptors that are involved in the JAK-STAT signaling pathway was observed following BPP-II treatment in hybridoma cells (supplemental Table S6). The potential role of BPP-II on Th1 and Th2 type cytokine responses was further confirmed in immunization experiments (Fig. 5B and supplemental Fig. S1B). The family of SOCS genes, including CISH, induced by STATs, were markedly induced in BPP-II-treated hybridoma (supplemental Table S4). It was predicted that that up-regulation of CISH would normally result in down-regulation of STAT3 and STAT5, and as expected, protein inhibitor of activated STAT3 (Pias3) was increased after BPP-II exposure. However, STAT4, STAT5a, STAT5b, and STAT6 factors which are involved in T cell activation and differentiation, proliferation, and B cell-mediated immune response, and cytokine-mediated signaling (Fig. 3 and supplemental Table S5), were up-regulated after BPP-II treatment. STATs are activated by tyrosine phosphorylation in response to cytokines and mediate many of their functional responses (29). Therefore, it was speculated that BPP-II regulates JAK-STAT signaling by STATs through various negative-feedback mechanisms, leading to T cell activation and various immune responses.
BF is the primary site of B cell lymphopoiesis in birds and is critical for antibody production and normal development of B lymphocytes accompanied by expression of a series of cell surface molecules and modulation by regulatory factors (3, 4, 29). In humoral immune responses, B cell differentiation and development are well characterized. In the bone marrow, the differentiation from pro-B cells to immature B cells can be defined by several surface antigens. Immature B cells are stimulated to become mature B cells that circulate in the peripheral blood as naive B cells. In the peripheral lymphoid tissues, naive B cells differentiate into memory B cells, or plasma cells (30). However, little is known about the differentiation of B cells in response to BPP-II treatment. In this paper, it was observed that BPP-II stimulated antibody production in immunized mice and splenocytes from immunized mice (Fig. 5A and supplemental Fig. S1A), and BPP-II treatment induced the down-regulation of Ms4a2 and the up-regulation of Fcer1a, which are subunits of the high affinity IgE receptor (31) involved in B cell-mediated immunity, regulation of humoral immune responses, and IgE receptor activity (Fig. 3 and supplemental Table S5). Enhancement by B cell receptor cross-linking involves wide involvement of the secondary messenger Ca2+ and simultaneous activation of multiple serine kinase pathways (32, 33). BPP-II regulated the expression of the CACNB4 and CACNA1I subunits of calcium channels, suggesting that BPP-II contributes to B cell receptor signaling through the regulation of calcium channels. These results indicated that BPP-II might modulate humoral immune responses; however, further research is required to elucidate the role and underlying mechanism of BPP-II in B cell differentiation.
BPP-II is a simple structured, low molecular weight peptide, which belongs to the hapten family of molecules (Fig. 1). It has been reported that hapten molecules used in isolation are poorly immunogenic vaccines (34). Therefore, it was found that mice immunized with BPP-II alone did not induce significant response (data not shown). However, the simple structural features of BPP-II do not influence its adjuvant activity. It was proved that BPP-II itself could induce antibody and cytokine production by splenocytes from immunized mice (supplemental Fig. S1), and microarray analysis showed that BPP-II stimulation regulated expressions of genes involved in T cell activation, humoral immune response, and cytokine production, suggesting that BPP-II might induce strong humoral and cellular immune responses in vitro through various immune-related signal activations. Furthermore, BPP-II functions as an immunomodulatory factor to induce AIV-specific immune responses (Fig. 5). These results suggested that BPP-II might be an immunomodulatory factor and could induce antigen-dependent humoral and cellular immune responses.
In the immune system, signal transduction pathways are functionally important for the appropriate development of properly selected T and B lymphocytes as well as in controlling responses to antigen by more mature cells (35, 36). These results illustrated the profound molecular basis of the regulation of BPP-II on the humoral immune and cellular immune responses.
The potential linking between biomaterials from the humoral central immune organ and antitumor immunity have yet never been elucidated. In this paper, expressions of various genes involved in various cancer pathways and apoptosis were shown to be regulated by BPP-II treatment (supplemental Table S6). It has been reported that the dysregulated expression of the Csf1r proto-oncogene is important for the resultant tumors survival in B cell-derived lymphoma cells (37). The Fas/Fas ligand (FasL) system is a key signaling transduction pathway of apoptosis in cells and tissues and plays an important role in immune privilege (38). Up-regulations of CSF1R and Fas suggest that BPP-II might contribute to antitumor defense (supplemental Table S4). Apoptosis plays an important role in homeostasis and tissue development (39). Up-regulation of Perp (TP53 apoptosis effector) and antiapoptotic molecule Bcl2l1 (40) following BPP-II treatment indicates dual functional roles in apoptosis.
The tumor suppressor p53 protein plays a crucial role following DNA damage (41). In this paper, BPP-II was shown to be identical to SAMD8. Sterile α motif (SAM) domains present in p63 and p73 family members, homologs of the tumor suppressor p53, might mediate negative regulation of p53-like activity (22). Homologous ubiquitin-conjugating enzyme E2 to BPP-II (supplemental Table S2), also known as ubiquitin-carrier proteins, participate in the ubiquitination process (21, 42). Ubiquitination plays a key role in regulating the stabilization and expression of p53 (43), and although the precise mechanisms are not fully understood, in posttranslational modification (44). It has been reported that various factors involved in the ubiquitination modification pathway influence p53 regulation. p53 ubiquitination and degradation are more complex, and MdmX, HAUSP, ARF, COP1, Pirh2, and ARF-BP1 continue to this pathway (43, 44). The results proved that BPP-II regulated gene expressions of the p53 signal pathway (supplemental Table S6) and enhanced p53 activity and expression (Fig. 6). However, the factors involved in BPP-II-stimulated p53 regulation remain to be elucidated, which will be major aspects in our subsequent research.
It was observed that high doses of BPP-II (5–50 μg/ml) selectively inhibited growth of tumor cells, but not normal cells (Fig. 7) by the MTT method. As an indicator of cell metabolic viability (11), the MTT assay represents a useful tool in estimating cell number and viability (12, 45). Clonogenic cell survival assay is used to assess reduction of the potential of individual tumor cells to form colonies (46). Flow cytometry provides a powerful and versatile approach to the measurement of cell death by the addition of various fluorescent nonvital DNA dyes (47). These three methods demonstrate different approaches to the analysis of cell proliferation and death. In this paper, BPP-II-mediated regulation of proliferation in normal and tumor cells was analyzed by the MTT method. In our further works, the function of BPP-II on proliferation or cell death by flow cell cytometry sorting or clonogenic assay will be investigated.
In this paper, it was found that although BPP-II induced p53 and Bax (Fig. 6), no inhibition effect on Vero cell viability was observed after BPP-II treatment (Fig. 7B). Apoptosis is tightly controlled by multiple conserved genes, including the Bcl-2 and caspase families. Bcl-2 protein represses the apoptotic death programs, whereas Bax (Bcl2-associated X protein) is an apoptosis-inducing protein (48), and the ratio of these two proteins determines cell fate following an apoptotic stimulus (49). Furthermore, Bcl-2 exhibits caspase-inhibiting activity downstream or aside from cytochrome c release to prolong cell survival after Bax induction (50). However, the effects of BPP-II on Bcl-2 protein and caspase proteins in Vero cells are unclear. Vero cell is derived from African green monkey kidney cell and exhibits characteristics different from tumor cells. It can be speculated that Vero cells share some regulation of antiapoptotic pathways, accounting for the observation that Vero cell viability was not significantly affected by BPP-II treatment, despite the observed increases in p53 and Bax protein levels. This phenomenon requires further investigation.
BPP-II was observed to play an antiproliferative role in tumor cells (Fig. 7). What is the probable effect of BPP-II as an adjuvant on antitumor immune responses? Tumors, unlike infectious organisms, do not induce effective innate immunity, and antitumor vaccines require adjuvant for efficacy (51). The competitive interaction between tumors and the immune system is highly complex (52). Prevention of tumor recurrence depends on the induction of a stably maintained population of specific CD8+ cytotoxic effector T cells (53, 54) that recognize antigen in the context of MHC class I expressed by the “target” cell (23, 55). In this paper, BPP-II was shown to exhibit similarity with MHC class I antigens (supplemental Table S2). BPP-II stimulation altered the CD8+ T cell phenotype (Fig. 5C) and reduced tumor cell proliferation (Fig. 7A). These results might provide information regarding adjuvant potential of BPP-II on antitumor immune responses in vivo. Our subsequent works will be an essential elucidation of the immunomodulatory functions of BPP-II in the induction of antitumor immune responses.
BF is the acknowledged central humoral immune organ responsible for B cell difference (3). These results suggest that BPP-II is a bursal-derived potential immunomodulatory peptide. Although the function of BPP-II in the immune induction requires further elucidation, information generated in this study elucidates the link between humoral and cellular-mediated immune responses, including antitumor, and provides some novel insights on the research of the humoral central immune system and on the effective immunotherapeutic strategies for treating human tumors and immune improvement.
We thank KangChen Bio-tech Inc. (China, Shanghai) for performing microarray analysis.
*This work was supported by Grant 200803020 from the National Agriculture Special Research Project for Nonprofit Trades, Ministry of Agriculture, China, and by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
2The abbreviations used are: