Structural consequences of ammonia binding to the manganese center of the photosynthetic oxygen-evolving complex: an X-ray absorption spectroscopy study of isotropic and oriented photosystem II particles

The structure and orientation of the manganese complex in NH3-treated photosystem II (PS II) membrane particles of spinach are being studied by X-ray absorption spectroscopy. On the basis of earlier work by our group, a structure for the tetranuclear manganese complex of PS II, which consists of two di-mu-oxo-bridged binuclear Mn units linked by a mono-mu-oxo group, has been proposed [Yachandra, V. K., et al. (1993) Science 260, 675-679]. The extended X-ray absorption fine structure (EXAFS) of the complex modified by NH3 binding in the S2-state is suggestive of an increase in the Mn-Mn distance of one of these units from 2.72 +/- 0.02 to 2.87 +/- 0.02 A, whereas the Mn-Mn distance of the second unit seems to be unaffected by NH3 treatment. The elongation of one binuclear center could result from the replacement of one bridging mu-oxo by an amido group. The lengthening of one Mn-Mn distance means that, by NH3 treatment, the distance degeneracy of the 2.7 A Mn-Mn EXAFS interaction is removed. Consequently, the orientation of individual binuclear units with respect to the membrane normal becomes resolvable by EXAFS spectroscopy of partially oriented PS II membrane particles. The angle between the normal of the PS II-containing membrane and the Mn-Mn vector is determined to be 67 degrees +/- 3 degrees for the 2.87 A distance and 55 degrees +/- 4 degrees for the 2.72 A distance. Only small effects on position, shape, and orientation dependence of Mn K-edge spectra result from NH3 treatment, indicating that the Mn oxidation state, the symmetry of the Mn ligand environment, and the orientation of the complex remain essentially unaffected in the annealed NH3 S2-state. Therefore, it seems likely that the angles determined for the ammonia-modified manganese complex are similar to the respective angles of the untreated complex. The structure of the manganese complex and its orientation in the membrane are discussed.

Structural features in heparin which modulate specific biological activities mediated by basic fibroblast growth factor

The biological activity of basic fibroblast growth factor (bFGF) is influenced greatly by direct binding to heparin and heparan sulphate (HS). Heparin-derived oligosaccharides have been utilized to determine the structural requirements present in the polymer that account for binding to bFGF. We had previously demonstrated that fragments > 6 mer can inhibit the interaction between cell surface heparan sulphate proteoglycan (HSPG) and bFGF, and bFGF-induced proliferation of adrenocortical endothelial (ACE) cells. In contrast, oligosaccharides > 10 mer can enhance the binding of bFGF to its high-affinity receptor or support bFGF-induced mitogenesis in ACE cells (Ishihara et al., J. Biol. Chem., 268, 4675-4683, 1993). We have extended these studies to size- and structure-defined oligosaccharides from heparin, 2-O-desulphated (2-O-DS-) heparin, 6-O-desulphated (6-O-DS-) heparin, carboxy-reduced (CR-) heparin and carboxy-amidomethylsulphonated (AMS-) heparin. Oligosaccharides from these polymers were fractionated on a bFGF-affinity column and were assessed as inhibitors or enhancers of specific bFGF-derived biological activities. The results of these studies indicate that both 2-O-sulphate and the negative charge of the carboxy group [L-iduronic acid (IdoA) residues] are required for specific interactions of heparin-derived oligosaccharides with bFGF and for modulation of bFGF mitogenic activity. In addition, the charge of the carboxy groups in uronic acids can be replaced by other functional groups with a negative charge, such as the amidomethyl sulphonate moiety described here.

Correlation between structure and magnetic spin state of the manganese cluster in the oxygen-evolving complex of photosystem II in the S2 state: determination by X-ray absorption spectroscopy

The structure of the manganese cluster in the S2 state with the g approximately 4 EPR signal (S2-g4 state) generated by 130 K illumination of photosystem II (PSII) membranes prepared from spinach has been investigated by X-ray absorption spectroscopy. The Mn X-ray absorption K-edge spectra of the S2-g4 state not only show a shift of the inflection point to higher energy from the S1 state but also reveal a different edge shape from that of the S2 state with the multiline signal (S2-MLS state). Extended X-ray absorption fine structure (EXAFS) studies of the Mn K-edge show that the structure of the Mn cluster in the S2-g4 state is distinctly different from those in the S2-MLS or S1 states. In the S2-g4 state, the second shell of back-scatters from the Mn absorber is found to contain two Mn-Mn distances of 2.73 and 2.85 A. We interpret this to indicate the presence of two nonequivalent di-mu-oxo-bridged Mn binuclear structures in the Mn cluster of the S2-g4 state. The third shell of the S2-g4 state at about 3.3 A also contains increased heterogeneity. By contrast, very little distance disorder was found to exist in the second shell of the S1 or S2-MLS states. A mechanism is proposed to explain these results in the context of our model for the Mn cluster and the EPR properties of the Mn complex in the S2 state.

Desensitization of adenylate cyclase in Chinese hamster ovary cells transfected with human thyroid-stimulating hormone receptor

Desensitization or decreased response to the same (homologous) or other stimuli (heterologous) is a well known process. Homologous desensitization to TSH has been demonstrated in normal thyroid tissue. Chinese hamster ovary cells (CHO) transfected with normal human TSH receptor (hTSHR) DNA, in contrast, have been reported not to desensitize. The purpose of our investigation was to determine whether CHO cells transfected with hTSHR desensitize in response to TSH and postreceptor stimulation. CHO cells were stably transfected with plasmid DNA containing hTSHR; nontransfected CHO cells served as the control. TSH (10 mU/ml), 5'-beta,gamma-imido-triphosphate [Gpp(NH)p; 0.1 mM], sodium fluoride (NaF; 10 mM), forskolin (10 microM), and (Bu)2cAMP (100 microM) were used to determine whether homologous or postreceptor heterologous desensitization of adenylate cyclase activity occurred in CHO-transfected cells. Intracellular cAMP accumulation was determined by RIA. Cells were incubated with TSH (to stimulate TSH receptor), Gpp(NH)p, NaF (to stimulate G-protein), forskolin (to stimulate adenylate cyclase activity), and (Bu)2cAMP (nonmetabolized cAMP analog). A second incubation was carried out with TSH (10 mU/ml). Maximal desensitization to either TSH or postreceptor stimulation was observed at 2 h. When transfected CHO cells were preexposed to TSH (10 mU/ml) for 4 h, even the smallest dose of TSH (0.001 mU/ml) caused desensitization. All substances that increased the intracellular cAMP concentration, such as TSH, Gpp(NH)p, NaF, forskolin, and (Bu)2cAMP, caused desensitization. The decrease in the cAMP response to TSH added in the second incubation was 63% less than the initial response to TSH or to postreceptor stimulation (P = 0.0001). In conclusion, desensitization of hTSHR-transfected CHO cells occurs in response to both receptor and postreceptor stimulation that increase cAMP levels. Because hTSHR transfected CHO cells desensitize, no specific thyroid factor(s) other than increased levels of cAMP is required.

Functional anatomy of forebrain auditory pathways in the budgerigar (Melopsittacus undulatus)

Interconnections of forebrain auditory and vocal control nuclei were mapped in the budgerigar using pathway tracing techniques. The anatomical results indicate four circuits by which auditory information may influence the vocal motor system: (1) direct auditory thalamic projections from nucleus dorsomedialis posterior (DMP) to both the neostriatal higher vocal center (HVC) and robust archistriatal nucleus (RA); (2) direct projections from a neostriatal projection field of DMP (i.e., MAN, the magnocellular nucleus of the neostriatum) to HVC and RA; (3) projections from DMP and other 'accessory' auditory thalamic nuclei to the ventral paleostriatum (VP), which in turn projects to MAN and RA; (4) projections to HVC from the lateral hyperstriatum ventrale (HV), which receives input from nucleus basalis (Bas) as well as from the oval nucleus of the HV (HVo), which receives direct input from RA. Lesion methods were used to evaluate the roles of auditory pathways in call learning and production. The results show that pathways associated with Bas are essential for call production in both adult and unfledged budgerigars, while VP efferents influence vocalization only in young, unfledged budgerigars. Lesions centered in either the primary auditory neostriatum (Field L2a) or the neostriatal area in receipt of Field L input (the ventrolateral neostriatum intermedium or NIVL) did not affect vocalization in juvenile or adult budgerigars.

Increased phospholipase C activity in neoplastic thyroid membrane

The phospholipase C (PLC)-protein kinase C (PKC) signal transduction pathway appears to be important for cellular growth of many normal and neoplastic tissues. Because alterations in the thyroid-stimulating hormone (TSH) receptor-adenylate cyclase-protein kinase A system in some thyroid tumors do not correlate with tumor size, invasiveness, or metastatic potential, we studied the PLC activity in both normal and neoplastic thyroid tissues from 11 patients. Five of these patients had follicular adenomas and 6 had papillary carcinomas. An 8,000 x g membrane fraction and a 105,000 x g cytosol fraction were prepared from the normal and neoplastic human thyroid tissues. PLC hydrolyzes phosphatidylinositol, 4,5-diphosphate (PIP2) to diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). Phospholipase C activity was determined measuring the hydrolysis of [3H]-PIP2. The activity of PLC in the neoplastic thyroid tissue membrane fraction (20.91 +/- 2.28 nmol PIP2 hydrolyzed/mg protein/120 min) was higher than that in normal thyroid membrane (14.27 +/- 0.82) (p < 0.05). In contrast, PLC activity was similar in the neoplastic (16.12 +/- 0.86 nmol PIP2 hydrolyzed/mg protein/120 min) and normal (16.66 +/- 0.60) cytosol. There was no difference between PLC activity in the membrane fraction from adenomas (21.21 +/- 3.71 nmol PIP2 hydrolyzed/mg protein/120 min) when compared with thyroid carcinomas (20.67 +/- 3.14). Neoplastic thyroid membranes have greater PLC activity than that found in normal thyroid membranes from the same patients. Although PLC activity in benign and malignant thyroid membranes was similar, the increased PLC activity in thyroid neoplasms may be responsible for or contribute to the enhanced growth of some thyroid tumors.

Rapid plasmid insert amplification with polymerase chain reaction

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Tumor-specific immunity induced by somatic hybrids. I. Lack of relationship between immunogenicity and tumorigenicity of selected hybrids

Hybrid clones were derived from fusion of TEPC-15 plasmacytoma cells of BALB/c mice with mouse L cells of C3H origin. The morphology, tumorigenicity, and immunogenicity of three representative clones were extensively studied. One clone (LTC-1) showed a morphology intermediate to that of either parental cell and possessed the highest tumorigenic and immunogenic properties. The other two clones displayed a "flat" morphology which differed significantly from that of either parent. One of these two, LTC-4, eventually induced tumors in some (BALB/c X C3H)F1 mice but failed to stimulate protective immunity against TEPC-15 tumor cells in BALB/c mice. The other hybrid clone, LTC-2, has a "very flat" morphology and did not induce tumors, although it was capable of stimulating a significant level of tumor immunity. Histologically, all the tumors induced by hybrid cells were fibrosarcomas rather than plasmacytomas. These results indicate that the morphology of hybrid cells may be correlated with the tumorigenicity as well as the histologic appearance of tumor. In addition, the degree of tumorigenicity of individual hybrid clones does not correspond to their immunogenicity in the host, suggesting that major antigens responsible for immunogenicity may not play an important role in induction of tumors.

Karyotypic abnormalities and clinical aspects of patients with multiple myeloma and related paraproteinemic disorders

Karyotypic abnormalities were detected in the malignant cells of 6 of 18 patients with multiple myeloma (MM). Six patients with benign monoclonal gammopathy, one with amyloidosis of immunoglobulin origin, and two with Waldenström's macroglobulinemia had normal karyotypes. All six MM patients with aneuploidy were in a group of 10 patients in an accelerated or relapse phase of their disease and four had high serum paraprotein levels (7.92, 6.24, 6.80, and 4.24 g/dl, respectively) when their abnormal karyotypes were detected. Five of the 6 MM patients with aneuploidy had received prior chemotherapy. Aneuploidy was not observed in 8 stable MM patients. Abnormalities of chromosome 14 were present in all 6 patients, with a 14q+ marker in 5 and loss of No. 14 in 1. A translocation between Nos. 11 and 14 was found in aneuploid cells of 2 patients who had plasma cell leukemia (PCL). However, the break point in the long arm of No. 11 differed in the 2 patients. A gain of Nos. 5, 9, and 11 was seen in 3 patients, a gain of No 1 in 2, and rearrangements of No. 1 in 5 MM patients, including all 4 who had a 14q+ marker chromosome initially. A deletion of chromosome 6 at band q25 was detected in 2 MM patients and a pericentric inversion of No. 6 (6p21 to 6q13) was seen in the patient with PCL. Three of 4 MM patients had a nonrandom loss of one chromosome 8. Two other MM patients, who were treated with melphalan and prednisone, developed acute nonlymphocytic leukemia (ANLL) 2+ and 4+ years after the diagnosis of MM. Marrow cells of one patient showed a 5q- chromosome and a constitutional translocation involving Nos. 13 and 14 during the preleukemic stage; during the leukemic phase, the karyotype evolved to 50 chromosomes including extra chromosomes 1, 6, 8, 10, and 21 and a missing 7, in addition to the originally detected 5q- and the 13/14 translocation. The peripheral blood from the other patient was hypodiploid, with a missing chromosome 7 and a translocation between 3q and 9p. These patterns of chromosome change resemble those of ANLL rather than MM and are similar to the changes seen in ANLL after treated malignant lymphoma.

Murine leukemia cell hybrids: the quantity of TL antigens expressed by parental and hybrid cells fails to correlate with their sensitivity to TL antibody and complement

The quantity of thymus-leukemia (TL) antigens expressed by murine leukemia cells is significantly greater than that expressed by somatic hybrids of such cells. Based upon the results of 125I-lactoperoxidase labeling and antibody absorption procedures, and corrected for size differences between the two cell types, the quantity of TL antigens expressed by RADA-1 cells, a radiation-induced murine leukemia cell line of strain A/J mice, is approximately 5.0 times greater than that of somatic hybrids of RADA-1 and LM(TK)- cells. LM(TK)- cells are a thymidine kinase-deficient TL(-) mouse fibroblast cell line. The quantity of TL antigens expressed is related only in part to their susceptibility to lysis by TL antibodies and guinea pig complement (GPC). RADA-1 cells resist lysis. The quantity of TL antigens expressed by RADA-1 cells is analogous to that formed by nonneoplastic thymocytes obtained from F1 hybrids of two strains of TL(+) and TL(-) mice; cells from both strains are sensitive to TL antiserum and GPC. ASL-1 cells, a spontaneously occurring leukemia cell line of A/J mice, express TL antigens in significantly higher quantities than any of the cell types examined. Exposed to TL antisera, the quantity of TL antigens of ASL-1 cells, but not that of hybrid cells, gradually diminishes. ASL-1 cells convert over a 6-h period of exposure to antibody and guinea pig complement (GPC) resistance; hybrid cells remain sensitive. However, ASL-1 cells converted to TL antibody and GPC resistance continue for a time to express TL antigens in quantities similar to that of sensitive F1 thymocytes and resistant RADA-1 cells. RADA-1 X LM(TK)- hybrid cells, which are sensitive to TL antibodies and GPC, express the lowest quantities of TL antigens of any of the cell types examined. It is likely that differences in the quantities of TL antigens expressed by different cell lines reflect genetic mechanisms controlling TL antigen expression. The failure of TL antisera to affect the quantities of TL antigens expressed by hybrid cells is taken as an indication that genetic controls governing antigen expression may be distinguished from those involved in regulating responsiveness to specific antiserum.

Detection of thymus leukemia antigens on the surface membranes of murine leukemia cells resistant to thymus leukemia antibodies and guinea pig complement

The thymus leukemia (TL) antigens of ASL-1 murine leukemia reversibly disappeared from the membranes of cells exposed to TL antisera; the cells acquired resistance to fresh TL antiserum and complement (antigenic modulation). Three independent methods, however, indicated that the acquisition of complement resistance preceded the complete disappearance of TL antigens from the cell surface. Modulated cells reduced known titers of TL antisera by absorption; they stained positively in immunofluorescence studies involving TL antibodies and fluorescence-labeled rabbit anti-mouse immunoglobulin. TL antigens labeled previously with 125I were recovered by immunoprecipitation from cellular extracts prepared with nonionic detergent. Continued exposure of the cells to TL antiserum led to virtually complete disappearance of the antigens. Similar results were obtained for RADA-1 cells, another murine leukemia that forms TL antigens, although in this instance the cells were resistant to the cytolytic effects of TL antisera and guinea pig complement (GPC) without prior exposure to TL antibodies. The density of TL antigens remaining on the surface of different TL(+) cell types failed to correlate with resistance to TL antibodies and GPC. Cells from F, hybrids of TL(+) and TL(-) mouse strains formed TL antigens and were susceptible to TL antibodies and GPC even though the density of TL antigens formed by the susceptible cells was less than the density of TL antigens formed by modulated cells. Stable somatic hybrids of RADA-1 cells and TL(-) cells formed TL antigens at lower density than did RADA-1 cells and lysed in the presence of aliquots of the TL antisera and GPC used in previous tests.

Resistance to murine leukemia in mice receiving simultaneous injections of syngeneic hybrid and parental neoplastic cells

(A/J x C3H/HeJ)F1 mice (H-2a/k) exhibit no apparent resistance to the malignant proliferation of ASL-1 leukemia cells (H-2a), a spontaneously occurring neoplasm of A/J mouse origin. The period of survival after injections of ASL-1 cells is related to the number of cells introduced into the susceptible host. (A/J x C3H/HeJ)F1 mice receiving viable ASL-1 cells along with ASL-1 x LM(TK)- somatic hybrid cells (H-2a/k), syngeneic with the recipients, survived for statistically significant (p less than 0.001), longer periods than did mice receiving ASL-1 cells alone. Mice receiving ASL-1 and ASL-1 x LM(TK)- hybrid cells together survived for approximately twice as long as animals receiving ASL-1 cells alone. Mice receiving ASL-1 cells followed by hybrid cells survived for longer periods as well, but the therapeutic effects were less successful. ASL-1 x LM(TK)- hybrid cells are rejected after forming localized tumors by the immunocompetent F1 mice. In some instances, F1 animals previously rejecting hybrid cells and challenged subsequently with ASL-1 cells along with a second injection of hybrid cells survived indefinitely. The passive transfer of partial immunity to the leukemic cells could be achieved by injections of spleen cells from F1 mice that had rejected hybrid cells previously. Similar results were obtained for (A/J x C3H/HeJ)F1 mice receiving RADA-1 cells (H-2a), a radiation-induced leukemic line of A/J origin, along with RADA-1 x LM(TK)- hybrid cells (H-2a/k). The partial immunity induced by these lines of hybrid cells was cross-reactive: mice injected with ASL-1 cells and RADA-1 x LM(TK)- hybrid cells survived for prolonged periods, and vice versa.

The effect of specific antiserum on the metabolism of three membrane-associated antigens of ASL-1 x LM(TK)- hybrid cells

Fusion of ASL-1 cells, a murine leukemia forming thymus leukemia (TL) antigens, with LM(TK)- cells, a TL(--) murine cell line, resulted in a stable hybrid forming TL antigens. The hybrids failed to undergo modulation, the reversible dissappearance of TL antigens from the surfaces of the cells, stimulated by TL antiserum. Unlike ASL-1 cells, the rate of disappearance of the antigens from modulation negative hydrid cells was unaffected by TL antiserum. The t 1/2 of TL antigens of the hybrid was approximately 30 h. The t 1/2 of TL antigens of ASL-1 cells was 10 h in the presence of TL antiserum, 18 h in the absence of TL antiserum. The rate of metabolism of a putative tumor-associated antigen of ASL-1 cells formed by the hybrid was unaffected by exposure to specific antiserum, as was the metabolism of H-2 antigens formed by the cell types.

Resistance to murine leukemia in mice rejecting syngeneic somatic hybrid cells

(A/J X C3H/HeJ) F1 mice reject somatic cell hybrids of ASL-1 cells (A origin) and LM(TK)- cells (C3H origin), but die from leukemia within 10 days after the inoculation of approximately 10(6) viable ASL-1 cells. Mice rejecting hybrid cells survive for prolonged periods after challenge with otherwise lethal numbers of ASL-1 cells. The hybrid cells, rejected by syngeneic F1 recipients, retained their oncogenic potential as determined by the appearance and progressive growth of tumors in immunologically deficient nu/nu mice injected with the cells. Similar results were obtained using hybrids of a radiation-induced cell line (RADA-1), maintained by serial transfer in strain A mice and LM(TK)- cells. Syngeneic mice injected with RADA-1 X LM(TK)- cells failed to form tumors. Mice rejecting RADA-1 X LM(TK)- hybrid cells survived for prolonged periods after challenge with otherwise lethal numbers of RADA-1 cells.

Complement sensitivity of somatic hybrids of a complement-resistant murine leukemia cell line

RADA-1 cells (H-2a), a murine leukemia cell line maintained by serial transfer in histocompatible recipients expressing thymus-leukemia (TL) 1, 2, 3 antigenic determinants, resisted the cytotoxic effects of guinea pig complement (GPC) and TL antiserum. The cells expressed a lower density of TL antigens than did ASL-1 cells, another TL(+) leukemia cell line expressing the same determinants and susceptible to complement (C)-mediated lysis. Stable somatic cell hybrids of RADA-1 cells and LM(TK)- cells (H2k), a TL(minus) thymidine kinase-deficient mutant of mouse L cells, were selected in hypoxanthine-aminopterin-thymidine medium. The hybrid cells expressed the H-2 antigens of both parents and shared a hybrid karyotype. They formed TL 1, 2, 3 antigens as determined by immunofluorescence, mixed hemagglutination methods, the direct isolation of TL antigens from Nonidet P40 extracts of the cells, and the capacity of the cells to reduce by absorption the known titers of TL antiserum. These hybrid cells lost the capacity to resist lysis by TL antiserum and GPC. They were susceptible to the cytotoxic effects of TL 1, 2, 3; TL 2; OR TL 1, 3 antiserum and GPC, even though the density of TL antigens associated with the cells was approximately 25% of their resistant RADA-1 parental cells. These results indicated that the mechanism of resistance to C-mediated lysis was genetically separable from the expression of TL antigens by the cells and that the susceptibility of the cells to the cytotoxic effects of antiserum was related only in part to the density of TL antigens expressed by the cells.

Detection of a TL(+) murine leukemia cell line that resists the cytotoxic effects of guinea pig complement and specific antiserum

RADA-1 cells [H-2a thy-1b; thymus-leukemia (TL) 1, 2, 3], a radiation-induced murine leukemia cell line maintained by serial transfer in histocompatible recipients, resisted lysis by guinea pig complement (GPC) and TL 1, 3; TL 2; or TL 1, 2, 3 antiserum. The cells expressed TL antigenic specificities as determined by indirect fluorescent antibody methods, the direct isolation of TL antigens from the cells, and the capacity of the cells to reduce known titers of TL antisera. GPC was consumed to the same extent during the reaction of resistant cells and TL antisera as occurred in the reaction of sensitive cells (killed under similar conditions) and TL antisera. RADA-1 cells were not nonspecifically resistant to complement (C)-mediated lysis; they were killed in the presence of H-2a antiserum and GPC. The TL antisera contained antibodies for TL determinants. They stimulated the C-mediated lysis of ASL-1 cells (TL 1, 2, 3) and thymocytes from strain A mice (TL 1, 2, 3). The TL antigens of resistant RADA-1 cells underwent antigenic modulation, the reversible disappearance of TL antigens from the cells stimulated by specific antiserum. After the cells were treated with neuraminidase, they became susceptible to the cytotoxic effects of aliquots of the same TL antisera and GPC used previously.

Somatic hybrid of thymus leukemia (

A somatic hybrid of ASL-1 leukemia cells [H-2a, thymus leukemia (TL)1,2,3, Thy-1b] and LM(TK)-cells [H-2-k, TL(-), Thy-1 (-), thymidine kinase deficient] was formed with the aid of inactivated Sendai virus. The selection of hybrid cell clones was facilitated by the inability of ASL-1 cells to grow in vitro and of LM(TK)-cells to survive in hypoxanthine/amethopterin/thymidine medium. The H-2 antigens of both parental cells were formed in approximately equivalent amounts by the hybrid cells, and they possessed a hybrid karyotype. As determined by five independent experimental procedures (antibody and complement-mediated cytotoxicity tests, the reduction of specific antibody activity of antiserum of known titer, immunofluorescent tests, mixed hemagglutination tests, and their direct isolation), TL antigens but not Thy-1 antigens were formed by the hybrid cells. TL antigens of the hybrids failed to undergo modulation under conditions leading to the modulation of TL antigens of parental ASL-1 cells. Modulation was attempted with TL 1,3, TL 2, or TL 1,2,3 antisera of high titer. thybrid cells were incubated for up to 30 hr in medium with TL antisera. Both direct and indirect immune methods were attempted. These results indicate that cellular mechanisms controlling the expression of TL antigens may be distinguished from the capacity of the cells to undergo modulation.