How Human Antibodies Block the Immune System's Off-Switch

Abgenix's 1999 patent on fully human monoclonal antibodies that bind to CTLA-4, a protein brake on the immune system, allowing T-cells to stay active and attack cancer cells.

What it covers

This patent claims a fully human monoclonal antibody that targets CTLA-4, a receptor on T-cells that acts as an off-switch for the immune system. According to Claim 1, the antibody uses a specific human gene sequence (VH 3-33 family) with key amino acid mutations to bind tightly to CTLA-4. Claim 4 specifies that the antibody must bind with high affinity (at least 10^-9 M) and be highly selective—at least 100 times more attracted to CTLA-4 than to similar proteins like CD28 or B7-2. By binding to CTLA-4, the antibody blocks it from interacting with its natural partners (B7-1 and B7-2), which prevents the immune system from shutting down. An example of its action is boosting the production of Interleukin-2 (IL-2), a signaling molecule that tells T-cells to multiply and fight.

What it doesn't cover

• —Does not cover antibodies derived from mice or other non-human animals that have not been fully humanized.
• —Does not cover antibodies that bind to CTLA-4 but also cross-react with lower mammals like mice, rats, or rabbits.
• —Does not cover antibodies with low binding affinity weaker than 10^-9 M or poor selectivity less than 100:1 over CD28.
• —Does not cover therapies that target the CTLA-4 pathway using non-antibody molecules, such as small-molecule drugs or soluble receptor proteins.

The clever bit

Instead of just finding any antibody that binds CTLA-4, the inventors engineered a fully human antibody that is highly selective against closely related proteins like CD28. This selectivity is crucial because CD28 is the 'on-switch' for T-cells; blocking it would accidentally shut down the immune response instead of boosting it.

Why it matters

This patent represents a foundational step in cancer immunotherapy. By blocking CTLA-4, it paved the way for drugs like tremelimumab to treat cancers like mesothelioma and non-small cell lung cancer by keeping the patient's own immune system active against tumors.

Real-world examples

• 1.Tremelimumab (Imjudo), an FDA-approved immunotherapy drug for liver and lung cancer.
• 2.XenoMouse technology platforms used to generate fully human antibodies.