Biochemical analysis of CTLA-4 immunoreactive material from human blood.
BACKGROUND
CTLA-4 was initially described as a membrane-bound molecule that inhibited lymphocyte activation by interacting with B7.1 and B7.2 molecules on antigen presenting cells. Alternative splicing of mRNA encoding the CTLA-4 receptor leads to the production of a molecule (sCTLA-4) that lacks a membrane anchor and is therefore secreted into the extracellular space. Despite studies finding that people with autoimmune disease more frequently express high levels of sCTLA-4 in their blood than apparently healthy people, the significance of these findings is unclear.


METHODS
Molecules isolated from blood using CTLA-4 specific antibodies were analyzed with ligand binding assays, mass spectroscopy, and biochemical fractionation in an effort to increase our understanding of CTLA-4 immunoreactive material.


RESULTS
Mass spectroscopy analysis of the molecules recognized by multiple CTLA-4-specific antibodies failed to identify any CTLA-4 protein. Even though these molecules bind to the CTLA-4 receptors B7.1 and B7.2, they also exhibit properties common to immunoglobulins.


CONCLUSIONS
We have identified molecules in blood that are recognized by CTLA-4 specific antibodies but also exhibit properties of immunoglobulins. Our data indicates that what has been called sCTLA-4 is not a direct product of the CTLA-4 gene, and that the CTLA-4 protein is not part of this molecule. These results may explain why the relationship of sCTLA-4 to immune system activity has been difficult to elucidate.
Tector
Matt
M
Aurora St Luke's Medical Center, Transplant Research Lab, Milwaukee, WI 53215, USA. matt.tector@aurora.org
Khatri
Bhupendra O
BO
Kozinski
Karen
K
Dennert
Kate
K
Oaks
Martin K
MK
D000911:Antibodies, Monoclonal / Q000276:immunology; D000936:Antigen-Antibody Complex / Q000097:blood / Q000737:chemistry / Q000276:immunology*; D015703:Antigens, CD / Q000097:blood / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D018122:B7-1 Antigen / Q000276:immunology / Q000378:metabolism; D051940:B7-2 Antigen / Q000276:immunology / Q000378:metabolism; D001798:Blood Proteins / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D060908:CTLA-4 Antigen; D005591:Chemical Fractionation; D006801:Humans; D056747:Immunomodulation; D013058:Mass Spectrometry; D009157:Myasthenia Gravis / Q000097:blood / Q000276:immunology*; D011485:Protein Binding
D000911:Antibodies, Monoclonal; D000936:Antigen-Antibody Complex; D015703:Antigens, CD; D018122:B7-1 Antigen; D051940:B7-2 Antigen; D001798:Blood Proteins; D060908:CTLA-4 Antigen; C556706:CTLA4 protein, human