Synthetic PLAD protein alleviates symptoms of both acute and chronic arthritis in mice and could be the basis for a new arthritis drug for people, report scientists from the National Institute of Allergy and Infectious Disease ( NIAID ), part of the National Institutes of Health ( NIH ).
The protein prevents the assembly of a cell surface receptor, thus blocking transmission of chemical signals that lead to arthritis symptoms.

" This study opens a new research avenue to better understand and, perhaps, to treat rheumatoid arthritis, a condition that causes suffering in more than two million Americans," says NIAID director Anthony S. Fauci.

Investigators from NIAID's Laboratory of Immunology, led by Michael Lenardo, published their findings in the journal Nature Medicine.

The idea that the protein, called pre-ligand assembly domain protein or PLAD, might play a role in thwarting the joint inflammation characteristic of rheumatoid arthritis - one of the most common autoimmune diseases - grew out of their research on a very rare autoimmune disease called autoimmune lymphoproliferative syndrome ( ALPS ).

Previously, Lenardo and his colleagues showed that in ALPS a form of PLAD blocks a cell surface receptor and prevents a needed chemical signaling pathway from functioning correctly. In ALPS, the signal pathway interrupted by PLAD leads to disease symptoms. But, the scientists reasoned, PLAD might also be able to block a related cell surface receptor - one involved in passing signals that lead to inflammation. In theory, inhibiting this pathway might benefit people with rheumatoid arthritis, who suffer from excessive inflammation.

A key promoter of inflammation is a chemical called tumor necrosis factor alpha ( TNF-alpha ).
TNF-alpha starts a chemical chain reaction leading to inflammation by binding to two cell surface receptors, TNFR-1 and TNFR-2.
Naturally occurring PLAD helps both forms of TNFR assemble and prepare to receive TNF-alpha. Synthetic PLAD, the scientists hypothesized, would bind to its natural counterpart and prevent it from performing its usual task.

The scientists used a variety of techniques ( including injection of TNF-alpha ) to induce arthritis symptoms in mice.
Researchers also injected some of the animals with lab-made PLAD ( P60 PLAD ).

" We found that P60 PLAD protein powerfully inhibited the symptoms of TNF-alpha-induced arthritis," says Lenardo.

P60 PLAD also lessened the effects of arthritis induced by other means. Moreover, he adds, P60 PLAD appeared to inhibit disease symptoms in mice with established as well as acute arthritis.
The scientists did not detect any obvious toxicity in the PLAD-treated mice.

" We're very hopeful that this could be good news for arthritis sufferers," says Lenardo. In particular, the researchers are intrigued by P60 PLAD's apparent specificity: it seems to block the binding of TNF-alpha to TNFR-1, while allowing TNFR-2 to continue to function. This is important, notes Lenardo, because it may represent an advantage over some currently used arthritis drugs, which directly block TNF-alpha by binding to both TNFRs and thereby inhibit beneficial actions mediated by TNFR-2.

Source: National Institutes of Health, 2005