Clinical impact of radiolabeled anti-CD4 antibodies in the diagnosis of rheumatoid arthritis

Human rheumatoid arthritis (RA) is characterized by severe chronic synovitis with abundance of CD4-positive T-cells and macrophages in the inflamed synovial tissue. These cells likely play a central pathogenetic role in RA and experimental models of arthritis. CD4 is a surface molecule present on the helper/inducer subset of T lymphocytes and macrophages, although with a lower density on the latter. CD4+ T-cells/macrophages and their cytokine products, therefore, represent potential therapeutic and diagnostic targets in RA. CD4, a 55 kDa monomeric glycoprotein, binds as a T-cell coreceptor to conserved areas of the major histocompatibility complex II on antigen-presenting cells, and thereby participates in the formation of the immunological synapse and the provision of the so-called ""second signal"" required for full activation of T-helper cells. A specific diagnostic or therapeutic approach is the direct targeting of CD4+ T-cells by anti-CD4 monoclonal antibodies (mAbs). In addition to therapeutic clinical trials with anti-CD4 mAbs in RA, which have yielded only ambiguous results, anti-CD4 mAbs have also been developed and applied for diagnostic purposes. The studies thus far conducted in RA have focused on the following aspects: 1) comparison of anti-CD4 mAb imaging to the established early methylene diphosphonate (MDP) scan; 2) biodistribution/ pharmacokinetics studies; and 3) specificity of joint imaging with anti-CD4 mAbs in comparison to control immunoglobulins with irrelevant specificity. The available results in RA and arthritis models show that 99mTc-anti-CD4 mAbs are well-suited to actively image diseased joints, and clearly allow more specific imaging than 99mTc-MDP or control immunoglobulins. Because effective treatment is known to reduce the density of CD4+ cells in the inflamed synovial membrane, diagnostic methods targeted to CD4 warrant further attention, also for early diagnosis of clinically silent joints, precise description of the cellular infiltrates, and monitoring of anti-rheumatic therapy.