As the first official post for Guillaume’s Mad mAbs, it seemed only fitting to provide an overview of the first approved monoclonal antibody: Orthoclone OKT3® (muromonab). OKT3, or Ortho Kung T-cell #3, was originally developed by Patrick Kung at Ortho Biotech in 1979 using a highly similar hybridoma-based technique George Köhler and César Milstein had developed a couple of years prior.

OKT3 is a mouse IgG2a antibody targeting the human cluster of differentiation 3 (or CD3) surface protein. Contrary to popular belief, the number 3 in OKT3 is not named after its target, but by the fact that the number 3 designated it as the third antibody listed in Kung’s lab notebook; CD3 would not be referred to as such until 1982 during the first Human Leukocyte Differentiation Antigen Workshop which was held in Paris, France.

The CD3 protein is specific for T cell lineage cells and is present at all stages of T cell development. CD3 is affiliated with the T cell antigen receptor (TCR), acting as a co-receptor in the CD3-TCR complex. Antigen-presenting cells (APCs), such as dendritic cells and B cells, express major histocompatibility complex (MHC) class II molecules which interact with T cells by presenting them non-self antigens; such is the case with foreign particles of bacterial or viral origin as well as abnormal proteins acquired from cancerous cells.

The MHC-TCR-CD3 interaction leads to T cell activation, initiating a cell-mediated immune response (one of the first steps in launching an adaptive immune response). OKT3 works by preventing CD3 from interacting with T cell receptors (TCRs), thus diminishing the likelihood of an immune activation. At certain concentrations, anti-CD3 antibodies have been shown to induce T cell proliferation. However, at higher concentrations, which is the case for OKT3, those antibodies have been shown to deplete the number of CD3+ cells from circulation.

In 1986, OKT3 was approved by the US Food and Drug Administration (FDA) for preventing rejection of solid-organ transplantations (in particular kidney transplants). The same principle applies here, where non-self proteins from a donor organ are recognized and processed by the recipient’s antigen-presenting cells, eliciting an immune reaction in the form of T cell activation, and oftentimes leading to organ rejection. Up until this point, polyclonal antibodies had been the therapy of choice, however, thanks to its monoclonal nature, OKT3 provided a more specific and predictable patient response.

Over the years sales began to diminish due to the immunogenicity of the drug and the side effects experienced by the patients receiving the treatment. Development of so-called Anti-Drug-Antibodies (ADAs), more specifically in the form of Anti-Mouse-Antibodies (AMAs), led to the formation antibody-immune-complexes, ultimately impacting the pharmacokinetics of OKT3. Immune reactions in the form of cytokine release syndrome lead to unfavorable symptoms (headaches, fevers, chills, nausea…) experienced by patients. Moreover, viral and bacterial based opportunistic infections ensued due to the T cell depleting and immunosuppressive nature of the drug.

More recently, in the late 1990s, interleukin 2 receptor (IL2R) targeting monoclonal antibodies such as Simulect® (basiliximab) and Zenapax® (daclizumab) became first line treatments for organ transplants and have since taken over the majority of the global organ transplant market. Following the MHC-TCR-CD3 interaction, T cells require other costimulatory signals for optimal T cell activation. The extensive molecular crosstalk between immune cells leads to signaling cascades within T cells which result in the expression of IL2R on their cell membranes. Further activation, via autocrine IL2 production, confirms the solicited immune activation.

Both basiliximab and daclizumab directly compete with the IL2 cytokine from binding to its receptor on the T cell surface, thus inhibiting receptor activation and ultimately preventing T cells from proliferating and differentiating. Both antibodies have shown to be less immunogenic than OKT3 (predominantly due to their chimeric and humanized nature respectively). When directly compared to OKT3, both antibodies not only led to lower chances of organ rejection and superior organ functions following surgery, but both have led to fewer opportunistic infections experienced by patients and, in time, have increased their quality of life.

As luck would have it, in 2008, Ortho Biotech would go on to merge with Centocor Biotech (a wholly owned subsidiary of the biopharmaceutical behemoth Johnson & Johnson). Two years later, OKT3 would be pulled from production, ending its 24 years of manufacturing, and in 2011 Centocor Ortho Biotech would go on to be renamed and from then on be known as Janssen Biotech.

Over one hundred diagnostic and therapeutic antibodies have been approved since 1986, but it was this specific approval that fueled the spreading wildfire of what would become known as the monoclonal antibody industry.

Guillaume Trusz

Author Guillaume Trusz

Guillaume Trusz received his B.S. in Molecular, Cell, and Developmental Biology from the University of California, Los Angeles (UCLA) in 2015 and his M.S. in Biomedical Imaging from the University of California, San Francisco (UCSF) in 2018. Prior to working as an Associate Scientist in the Discovery Immunology Group at Curia, Guillaume contributed to various academic and industry related research projects pertaining to small molecules, nanoparticles, as well as biosimilars.

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Join the discussion 4 Comments

  • Kristin Rider says:

    Great resource, thank you!

  • Jeffrey Bluestone says:

    Good news is that anti-CD3 mAbs are not dead yet! We developed a second generation OKT3, now called Teplizumab, Working with the very talented group at Ortho (Linda Jolliffe and Bob Zivin, and John Adair at Celltech) in 1987, using COSMID technology, the original OKT3 was altered to reduce immunogenicity (so-called humanization) and changed the Fc of the Ig portion to an Fc receptor non-binding IgG1 (mutations at amino acids 234,235). The resulting mAb has been in development the last 30 years for the treatment of T1D culminating in a publication last year in NEJM ( demonstrating that the Drug can delay the development of clinical diabetes in patients destined to develop T1D. The drug is also in a Phase III trial in patients with new onset T1D and has shown efficacy, without side effects or significant immunogenicity, in psoriatic arthritis and organ transplantation. So, perhaps in the near future, an anti-CD3 may be back on the market.

    As a disclaimer, I have a financial interest in Teplizumab and sit on the BOD of Provention, the biotech company developing the drug.

    • Guillaume Trusz says:

      Dear Dr. Bluestone

      Thank you for your comment and for sharing this very exciting information. The NEJM publication you shared was quite an interesting read. If there is anything that I, the Discovery Immunology Team, or LakePharma in general can do to assist you with this antibody, please do not hesitate to contact us. In the meantime, I wish you the best of luck with Teplizumab. We look forward to having the pleasure of one day adding this antibody to the long-standing list of approved monoclonal antibodies under the History tab of

      All the best,


  • Like!! Great article post.Really thank you! Really Cool.

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