Overview

The SP263 assay is a rabbit monoclonal antibody used in immunohistochemistry to quantify PD-L1 on tumor cells, and its clinical relevance hinges on a precise detail: which antibody clone you use changes what you see under the microscope. SP263 differs from competitors such as 22C3 and 28-8 in binding epitope and staining intensity, yet shows high concordance with 22C3 for tumor-cell scoring.

How the SP263 Antibody Works at the Molecular Level

SP263 is an IgG1 rabbit monoclonal antibody directed against the extracellular domain of human PD-L1. Applied to formalin-fixed, paraffin-embedded sections, it binds PD-L1 on tumor cell membranes and, in some tissues, tumor-infiltrating immune cells. The bound antibody is visualized through an enzyme-linked detection system — typically a horseradish-peroxidase conjugate and DAB — producing a brown membrane signal.

The assay's behavior is set by antibody specificity and dilution. SP263 generally produces more intense, cleaner membrane staining than some clones, which is why it was validated across indications including urothelial carcinoma, where immune-cell scoring matters. PD-L1 expression is heterogeneous within a tumor, so the assay's value depends on a standardized scoring method.

Reading the Tumor Proportion Score (TPS)

The Tumor Proportion Score quantifies PD-L1 on tumor cells only: the percentage of viable tumor cells showing partial or complete membrane staining at any intensity. In non-small cell lung cancer, the decisive thresholds are 1% and 50%. Below 1% is negative; 1% to 49% is low-positive, often prompting combined chemotherapy and immunotherapy; 50% or higher qualifies for single-agent PD-1/PD-L1 blockade.

Accurate TPS reading demands strict field selection. Pathologists score only viable tumor cells, exclude necrosis, and count only membrane staining — cytoplasmic-only staining does not count. SP263 can cause over-scoring, so laboratories should anchor readers to a validated training set.

Combined Positive Score Versus TPS: When Each Applies

The Combined Positive Score (CPS) extends the count beyond tumor cells: PD-L1-positive tumor cells plus PD-L1-positive immune cells, divided by total viable tumor cells, multiplied by 100. Because the denominator stays the tumor-cell count, CPS can exceed 100 when immune-cell staining is dense. CPS is the standard readout in gastric, esophageal, cervical and urothelial cancers.

Choosing between TPS and CPS is not optional — they are validated in different tumor types. In lung cancer, TPS is established; in many upper gastrointestinal and genitourinary cancers, CPS is required. SP263 interpretation must match the indication's approved metric.

FAQ

Q: What does the SP263 antibody bind and how is it detected? A: SP263 is a rabbit monoclonal antibody against the extracellular domain of PD-L1. It binds PD-L1 on tumor and immune cell membranes and is visualized with an enzyme-linked, DAB chromogenic detection system.

Q: What is the difference between Tumor Proportion Score and Combined Positive Score? A: TPS counts only PD-L1-positive tumor cells as a percentage of all tumor cells, used mainly in lung cancer with 1% and 50% thresholds. CPS also adds PD-L1-positive immune cells to the numerator, with a fixed tumor-cell denominator, and is used in gastric, esophageal, cervical and urothelial cancers.

Q: Why does antibody clone choice matter if the clones are concordant? A: Clones differ in epitope and staining intensity. While SP263 and 22C3 show high concordance for tumor-cell scoring, standardization matters: the scoring metric must match the indication and validated assay to stay comparable to trial data.