Interleukin-4 induces lipogenesis in porcine endothelial cells, which in turn is critical for induction of protection against complement-mediated injury.

Interleukin (IL)-4 has been shown to induce protection in porcine vascular endothelial cells (ECs) from killing by human complement. This protection is dependent on the PI3K/Akt signaling pathway. In this study, we investigated mechanisms downstream of Akt and found that activation of the lipid biosynthesis pathway is required for protection from complement in ECs treated with IL-4. Cells incubated with IL-4 for 48 hours contained increased fatty acids and phospholipids but cholesterol was not increased when compared with medium-treated controls. The transcription factor SREBP-1, which regulates fatty acid synthesis, was found to be activated in extracts of ECs incubated with IL-4 for 6 hours. Finally, induction of protection from complement killing with IL-4 was fully prevented by the presence of the SREBP inhibitor 25-OH cholesterol. This study showed that IL-4 induces lipid biosynthesis in porcine ECs through activation of SREBP-1 and that the activation of this pathway is critical for IL-4 to induce protection of porcine ECs from killing by human complement. Further study of these mechanisms may provide new strategies for the prevention of complement-mediated vascular injury as it occurs in xenograft rejection.

Comparison of linear and branched peptide forms (MAPs) in the induction of T helper responses to point-mutated ras immunogens.

The utility of multiple antigenic peptides (MAPs) for the induction of antibody and cellular immune responses in animal models has been demonstrated for a variety of peptide epitopes involved in human disease. However, little is known about immune responses to MAPs constructed with antigenic tumor epitopes, nor has peptide specificity in branched forms been addressed. A potentially important advantage of the MAP system over linear peptide immunogens for clinical applications is elimination of the need for a protein carrier with its associated toxicity and immunogenicity. Here, we examined cellular immune responses following in vivo administration of MAPs incorporating a 13-mer T helper epitope from point-mutated ras p21 (ras V12) and compared the potency of the responses to that of the linear peptide. The Gly --> Val mutation in position 12, which is associated with a range of human carcinomas, represents a useful system for evaluating the specificity of the immune response. In initial studies with the point-mutated linear peptide epitope, optimal in vitro proliferation responses were obtained following sc administration of the peptide in a squalane-containing adjuvant formulation. Comparative immunization studies using point-mutated MAPs bearing two, four, or eight branches were administered either in saline or in adjuvant. These studies showed that adjuvant was required for the induction of cellular immune responses using both linear and all three forms of branched peptides. Moreover, there was no apparent advantage of using any of the MAPs vs linear peptide when equivalent mass amounts were administered, i.e., the intensity of the immune response was no greater using any of the branched structures compared to the linear form. Specificity of the in vivo responses for both the linear and the MAP immunogens was demonstrated by the higher stimulation indices observed in vitro in the presence of the mutant ras V12 vs the normal ras G12 linear peptide. No apparent cellular immune response to the MAP core structure itself was observed. However, a nonspecific response to the two-branched MAP2G12 structure was observed in some assays, the nature of which is unknown at this time. This work represents the first reported investigation of a cellular immune response using MAP immunogens incorporating a tumor-specific peptide epitope and demonstrates that linear peptides are as efficient as three different MAP structures in the generation of specific T cell responses.

Biological properties of chimeric domain-deleted anticarcinoma immunoglobulins.

CC49 is a second-generation monoclonal antibody (MAb) that has high affinity for the tumor-associated pancarcinoma antigen tumor-associated glycoprotein-72. In clinical trials using gamma scanning, radiolabeled CC49 has facilitated the detection of more than 90% of carcinomas. We report here the development of a constant heavy-chain 2 (CH2) domain-deleted chimeric (c) CC49 MAb by transfecting an expression construct consisting of the CC49 murine variable region and a CH2 domain-deleted human IgG1 constant region into cCC49 kappa producing SP2/0 murine myeloma cells. As determined by SDS-PAGE, the intact cCC49 delta CH2 has a molecular weight of 153,000 and, under reducing conditions, molecular weights of 43,000 and 27,000. The plasma clearance and tumor-targeting properties of cCC49 delta CH2 were evaluated and compared with those of mouse/human chimeric forms cCC49 delta CH1 and intact cCC49. Previous studies have shown that the in vitro antigen-binding properties of cCC49 delta CH1 are similar to those of cCC49. Biodistribution studies reported here, using 131I-labeled cCC49 delta CH1 and 125I-labeled cCC49 in athymic mice bearing human colon carcinoma xenografts, demonstrated that both cMAbs localized to the tumor and cleared from the normal tissues similarly. However, in comparison with 125I-labeled cCC49, 131I-labeled cCC49 delta CH2 localized to tumors earlier and had a significantly lower percentage of the injected dose of cMAb/g (%ID/g) in normal tissues than cCC49. Immunoscintigraphy of 131I-labeled cCC49 delta CH2 and 125I-labeled cCC49 in athymic mice bearing human tumor xenografts demonstrated a clear image of the tumor by 24 h after i.v. administration of the delta CH2 cMAb versus the 72 h required for cCC49. Biodistribution studies using 177Lu-conjugated cCC49 delta CH1 and cCC49 showed no significant difference between the radiolocalization indices (% ID/g in tumor divided by % ID/g in normal tissue). 177Lu-conjugated cCC49 delta CH2, however, had lower % ID/g values in tumor xenografts and lower radiolocalization indices than either 177Lu-conjugated cCC49 delta CH1 or 177Lu-conjugated cCC49. Pharmacokinetic studies in non-tumor-bearing athymic mice using cCC49 delta CH1 and cCC49 revealed no significant difference between these cMAbs. However, the plasma clearance of cCC49 delta CH2 in non-tumor-bearing mice was significantly faster than that of cCC49. These results were similar when the cMAbs were labeled with either iodine or lutetium. In nonhuman primates, 131I-labeled cCC49 delta CH2 cleared significantly faster than 125I-labeled cCC49. The similar plasma clearance and tumor localization of cCC49 and cCC49 delta CH1 suggest that these two cMAbs may be used in similar clinical settings. However, because of the unique pharmacokinetics and tumor targeting of cCC49 delta CH2 versus cCC49 or cCC49 delta CH1, this chimeric immunoglobulin form may be useful in clinical settings that require efficient tumor targeting and rapid serum and whole-body clearance.

Biodistribution and preclinical radioimmunotherapy studies using radiolanthanide-labeled immunoconjugates.

Lutetium-177 (177Lu), samarium-153 (153Sm), and yttrium-90 (90Y) are members of the family of elements known as lanthanides or rare earths. Monoclonal antibody CC49, a murine immunoglobulin (Ig) G1, which is reactive with the tumor-associated antigen TAG-72, previously has been shown to react with a wide range of human carcinomas. The authors review here the comparative biodistributions of CC49 IgG and F(ab')2 fragments labeled with 177Lu, 153Sm, and 90Y using the bifunctional chelating agent PA-DOTA. The authors also review the results of a biodistribution study comparing iodine-125-labeled and 177Lu-labeled CC49 sFv, and the use of 177Lu-CC+9 IgG in an experimental therapy model. Chelation and conjugations gave similar yields, and the labeled proteins showed similar retention of immunoreactivity regardless of the isotope used for both IgG and F(ab')2. Biodistribution data obtained in athymic mice bearing LS-174T human colon carcinoma xenografts likewise showed no differences among the three radioisotopes for both IgG and F(ab')2. Femur uptake of radioactivity was lower than previously reported for other radiolanthanide immunoconjugates. Different metabolic patterns were observed for radioiodinated versus radiometal-labeled sFv, particularly in the kidney, where localization of the latter was increased dramatically. 177Lu-CC49 was found to delay the growth of established LS-174T human colon carcinomas in athymic mice at a single dose of 50 microCi. Elimination of established tumors was demonstrated over the observation period (77 days) using single administrations of 200 or 350 microCi. Dose fractionation experiments revealed that the mice tolerated 750 microCi (3 x 250 microCi, given weekly), whereas > 50% of the mice died after receiving a single administration of approximately 500 microCi. In isotype-matched control experiments, a large differential in the therapeutic effects was observed between 177Lu-labeled control antibody and CC49.

An improved linker for single-chain Fv with reduced aggregation and enhanced proteolytic stability.

The effects of linker length on binding affinity and degree of aggregation have been examined in the antifluorescein 4-4-20 and anticarcinoma CC49 single-chain Fvs. Longer linkers in the antifluorescein sFvs have higher affinities for fluorescein and aggregate less. A proteolytically susceptible site between Lys8 and Ser9, in the previously reported 212 linker has been identified. A new linker sequence, 218 (GSTSGSGKPGSGEGSTKG) was designed in which a proline was placed at the C-terminal side of the proteolytic clip site in the 212 linker. The CC49 sFv containing the 218 linker showed reduced aggregation and was found to be more stable to proteolysis in vitro, when compared to the CC49/212 sFv. The CC49 sFv with the longer 218 linker had higher affinity than CC49/212 sFv. An aggregated CC49/212 sFv sample had higher affinity than CC49/218 sFv. The CC49/218 and CC49/212 sFvs had similar blood clearances in mice, while the aggregated CC49/212 sFv remained in circulation significantly longer. In mice bearing LS-174T human colon carcinoma xenografts, the CC49/218 sFv showed higher tumor uptake than the CC49/212 sFv and lower tumor uptake than the aggregated CC49/212 sFv. The higher tumor uptake of the CC49/218 is most likely a result of its higher resistance to proteolysis. The higher affinity and higher tumor uptake of the aggregated CC49/212 sFv are most likely due to the repetitive nature of the TAG-72 antigen and the higher avidity of multivalent aggregates. When the sFvs were radiolabeled with a lutetium-chelate the CC49/218 sFv showed a lower accumulation in the liver and spleen compared to the aggregated CC49/212 sFv.

Preparation, characterization, and in vivo biodistribution properties of synthetically cross-linked multivalent antitumor antibody fragments.

Two new antibody forms of the general structure F(ab')n (n = 3 or 4) were prepared and tested in vivo as part of an ongoing search for antibody candidates with improved biodistribution properties for cancer immunotargeting applications. The novel multivalent antibody forms, called F(ab')3-x (tribody, 150 kDa, x = cross-linker) and F(ab')4-x (tetrabody, 200 kDa), were constructed through chemical cross-linking of Fab' subunits derived from murine CC49 IgG, a monoclonal antibody which recognizes the tumor-associated antigen TAG-72. Two new chemical reagents (trismaleimide 1 and tetramaleimide 2) were synthesized for use in cross-linking cysteine sulfhydryl groups present on the hinge region of Fab'. Homogeneous Fab' was prepared by mild reduction of F(ab')2 followed by selective reoxidation of interchain disulfide bonds, leaving a single hinge-region cysteine sulfhydryl group available for modification. For biodistribution studies, the parent F(ab')2 fragment was first radiolabeled via lysine amine modification using the isothiocyanate derivative of the 105Rh(BA-2,3,2-tet)Cl2 complex. Both new fragment forms were shown to retain antigen binding ability in vitro using a solid-phase immunoassay. Although isolated yields for F(ab')3-x and F(ab')4-x were low (18 and 4%, respectively), sufficient quantities were prepared for preliminary biodistribution studies in Balb/c mice and, in the case of F(ab')3-x, for a 5-day biodistribution study in tumor-bearing nude mice. A large proportion of the 105Rh-labeled F(ab')4-x was found to accumulate in the liver, possibly indicating an upper size limit for the in vivo use of cross-linked fragments. The biodistribution behavior of 105Rh-labeled F(ab')3-x in both Balb/c and nude mice was intermediate between that of IgG and F(ab')2 for all organs studied. Kidney localization was reduced, while blood circulation time and tumor accumulation were slightly increased, for the trivalent species compared with F(ab')2. The unique biodistribution profile of F(ab')3-x suggests the possible use of this multivalent fragment for in vivo tumor targeting applications.

Differential metabolic patterns of iodinated versus radiometal chelated anticarcinoma single-chain Fv molecules.

Genetically engineered single-chain Fvs (sFv) are defined as recombinant proteins composed of a variable light chain amino acid sequence of an immunoglobulin tethered to a variable heavy chain sequence by a designed peptide. Previous studies using iodine-labeled sFv, derived from the anticarcinoma monoclonal antibody CC49, showed that the 125I-sFv could efficiently target antigen-positive tumors in a human tumor xenograft model while demonstrating rapid plasma clearance and minimal uptake in normal organs. One of the issues we raised in the analysis of the iodinated sFv metabolic studies was whether similar metabolic patterns would be observed if the sFv were labeled with a radiometal. In the studies reported here, 125I-CC49 sFv and 177Lu-CC49 sFv were co-injected in mice bearing antigen-positive carcinoma xenografts. Both sFv forms showed similar tumor targeting and plasma clearance pharmacokinetics. The 177Lu-sFv, however, showed a greater uptake in liver and spleen and a much higher uptake in kidney. These studies thus demonstrate that despite their small size (M(r) 27,000), the metal-chelated sFv shows a metabolic pattern very different than that of the iodinated sFv, which is most likely due to retention of the metal by organs metabolizing the sFv.