MALA-2 (Murine Lymphocyte Activated Antigen-2) is a murine cell surface antigen that is detected at high concentration on activated, proliferating lymphocytes, but only weakly on resting lymphocytes. It is thought to play an important role in lymphocyte activation since the rat monoclonal antibody YN1/1.7.4 which recognizes MALA-2 is capable of inhibiting the mixed lymphocyte reaction. Considering the central role of lymphocyte activation to the generation and maintenance of the immune response, I undertook the purification and biochemical characterization of MALA-2. In these studies, MALA-2 was isolated and purified to homogeneity using immobilized YN1/1.7.4 monoclonal antibody and sodium docecylsulphate-polyacrylamide gel electrophoresis. Biochemical characterization studies revealed that MALA-2 is a Mr 95-100 kD glycoprotein containing a protein backbone of approximately 66 kD, and N-linked carbohydrate chains amounting to a Mr of approximately 35 kD. Two dimensional gel electrophoresis suggested that MALA-2 has an isoelectric point of 4.9. Although it was previously suspected that MALA-2 might be associated with the transferrin receptor on the cell surface, this was shown not to be the case on NS-1 cells. Additionally, ³²P-orthophosphate labelling of MALA-2 on NS-1 or MBL-2 cells could not be detected. Finally, the partial amino acid sequence of MALA-2 was determined by sequencing trypsin-generated peptides from purified MALA-2. Computer-assisted homology comparisons of the MALA-2 partial amino acid sequences with other known sequences showed that MALA-2 shared its most consistent homology with a class of proteins known as the immunoglobulin superfamily. Subsequent to this study, the partial amino acid sequences obtained within this study were used to construct oligonucleotide probes. These probes were used for the screening of cDNA libraries, facilitating the successful cloning of the MALA-2 gene. This, in turn, resulted in the identification of MALA-2 as the murine counterpart of the human ICAM-1 molecule, a protein known to play a significant role in intercellular adhesion and lymphocyte activation within the immune system. Significantly, results obtained from the biochemical characterization of MALA-2 carried out in this thesis have been confirmed by the subsequent nucleotide sequence data from the cloning of MALA-2. / Medicine, Faculty of / Pathology and Laboratory Medicine, Department of / Graduate

Human platelets respond to various macromolecules in the plasma. Uptake of specific ligands, and antibodies to various epitopes on the platelet plasma membrane, has been observed. The platelet canalicular system has been shown to be involved with this uptake. Recently, investigators have speculated on the role of endocytosis in platelets to account for the presence of plasma proteins such as fibrinogen and immunoglobulin within platelet organelles. Antibodies binding to cell-surface antigens on platelets can lead to a redistribution of these antigens. When antibodies, specific for platelet cell-surface receptors, bind to platelets they may either undergo endocytosis into intracellular vacuoles, or may merely become sequestered within the canalicular system of platelets. The present study investigated whether endocytosis occurs in platelets. Such a process would lead to the endocytic uptake of a fluid-phase marker and would involve internalization and recycling of cell surface membrane. A fluid-phase marker (FITC-dextran) was used to measure any constitutive endocytic activity. In addition, a suitable membrane marker was used to determine whether membrane internalization occurred. This involved a technique whereby radioactive galactose was covalently attached to cell-surface glycoconjugates. A monoclonal antibody to the platelet receptor, GPIIbIIIa, was used in conjunction with the membrane marker in order to determine if membrane internalization was involved during the subsequent redistribution of the receptor-antibody complex. Immunocytochemical techniques using electron-dense probes were employed to localise the sites to which this receptor-antibody complex became redistributed. In comparison with reported rates of endocytic uptake of fluid-phase marker in other cell types, no significant endocytic activity could be detected with platelets, after taking their relatively small volume into account. Similarly, membrane internalization was not detected with resting platelets. Following challenge of the platelets with anti-GPIIbIIIa antibody, no membrane internalization could be measured during redistribution of the receptor-antibody complex. The compartment to which the receptor-antibody complex was redistributed could be identified morphologically as the canalicular system. The present data provide evidence for a process of sequestration of receptor-antibody in the canalicular system of resting platelets. It remains possible that other mechanisms exist within the platelet system for uptake of extracellular material as this study dealt exclusively with the platelet response to a specific antibody. These results may have implications with respect to the interaction of platelets with anti-platelet antibodies in the normal state, as well as with clinical disorders involving elevated levels of platelet-associated IgG. As far as can be deduced from the available literature, these data represent the first use of a covalent membrane marker in conjunction with uptake of macromolecules to study endocytic events in human platelets.