CD45 Marker and Its Role in Immune Cell Regulation

CD45 is a crucial protein on most immune cells, functioning as a phosphatase that regulates signaling pathways controlling immune responses. Its widespread expression and significance make it a key focus in immunology and clinical diagnostics.

Structural And Biological Properties Of CD45

CD45, also known as protein tyrosine phosphatase receptor type C (PTPRC), is a transmembrane glycoprotein essential for cellular signaling. It consists of an extracellular domain, a single transmembrane segment, and an intracellular phosphatase domain. The extracellular portion is highly glycosylated, contributing to its variable molecular weight (180–240 kDa) based on post-translational modifications and isoform expression. This glycosylation affects receptor clustering and downstream signaling.

The intracellular domain contains two tandem phosphatase domains, with only the first being catalytically active. It dephosphorylates key tyrosine residues on Src family kinases, directly influencing signal transduction pathways that regulate immune cell activation, differentiation, and survival. The second phosphatase domain, though enzymatically inactive, stabilizes the protein and supports enzymatic function.

CD45 is exclusively expressed on hematopoietic cells, making it a key immune cell marker. Its expression levels and isoform composition vary by cell type and activation state, influencing immune responses. Alternative splicing of its extracellular domain generates multiple isoforms, each with distinct regulatory properties that modulate receptor interactions and activation thresholds.

Isoforms In Lymphocyte Subsets

CD45 exists in multiple isoforms due to alternative splicing of exons 4, 5, and 6 in its extracellular domain. These isoforms are differentially expressed across lymphocyte subsets, affecting signaling thresholds and functional properties.

T Cells

In T lymphocytes, CD45 isoform expression is linked to maturation and activation. Naïve T cells primarily express the high-molecular-weight isoform CD45RA, which includes all three alternatively spliced exons and is associated with a higher activation threshold. Upon antigen exposure, memory T cells express CD45RO, a lower-molecular-weight isoform that enhances T-cell responsiveness by facilitating more efficient T-cell receptor (TCR) signaling.

The transition from CD45RA to CD45RO distinguishes naïve from memory T cells. Studies in The Journal of Immunology show that CD45RO+ memory T cells respond more rapidly and robustly to antigens than CD45RA+ counterparts. This isoform shift optimizes adaptive immunity by ensuring a faster response to previously encountered pathogens.

B Cells

CD45 isoform expression in B cells is less dynamic than in T cells but still influences development and function. Immature and naïve B cells primarily express larger isoforms like CD45RA, associated with restrained activation. Upon activation and differentiation, smaller isoforms such as CD45RO become more prevalent, potentially enhancing signaling efficiency.

Research in Frontiers in Immunology suggests that CD45 isoform expression modulates B-cell receptor (BCR) signaling by regulating Src family kinases like Lyn. This affects activation thresholds, impacting antibody production and immune memory. While CD45 is not a primary marker for B-cell subsets, its role in BCR signaling underscores its importance in humoral immunity.

NK Cells

Natural killer (NK) cells express CD45, though their isoform distribution differs from T and B cells. Most NK cells predominantly express intermediate-sized isoforms, such as CD45RO and CD45RB, which support activation and cytotoxic functions. Unlike T cells, NK cells do not undergo a clear transition between CD45RA and CD45RO, but isoform variations correlate with functional states.

A study in The Journal of Leukocyte Biology found that CD45RO+ NK cells exhibit enhanced degranulation and cytokine production, indicating a more active functional state. CD45RB expression is linked to regulatory NK subsets that modulate immune responses. These findings highlight CD45’s role in fine-tuning NK cell activity, relevant for immune surveillance and NK cell-based immunotherapies.

Role In T-Cell Receptor Signaling

CD45 is critical for TCR signaling, regulating Src family kinases that initiate intracellular signaling cascades. It dephosphorylates inhibitory tyrosine residues on Lck and Fyn, key kinases in TCR activation. In resting T cells, Lck remains in a phosphorylated, inactive state. Upon antigen recognition, CD45 removes this inhibitory phosphate, activating Lck, which then phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) on the CD3 and ζ-chain components of the TCR complex. This phosphorylation recruits ZAP-70, amplifying downstream signaling.

CD45 isoform expression modulates signaling thresholds. Memory T cells, predominantly expressing CD45RO, have a lower activation threshold than naïve T cells expressing CD45RA. Research in Nature Immunology shows that CD45RO enhances TCR signaling efficiency by promoting sustained Lck activation, leading to faster responses.

Membrane localization further influences CD45’s role in signaling, as its distribution within lipid rafts affects receptor clustering. During TCR activation, CD45 is excluded from these regions to allow stable signal propagation. Disruptions in this mechanism can impair T-cell activation. Mutations in PTPRC, the gene encoding CD45, have been linked to defective TCR signaling in certain immunodeficiency disorders, emphasizing the importance of its spatial regulation.

Laboratory Methods Of Detection

CD45 detection relies on techniques that assess its expression across immune cells. Flow cytometry is the most widely used method, enabling rapid analysis of CD45 levels on individual cells. Fluorochrome-conjugated monoclonal antibodies target CD45 epitopes, allowing precise quantification. Multiparametric flow cytometry enhances characterization by assessing CD45 alongside other surface markers.

Immunohistochemistry (IHC) and immunofluorescence provide additional detection methods, particularly for tissue samples. These techniques use enzyme-linked or fluorescently tagged antibodies to visualize CD45 expression in histological sections. IHC is commonly used in diagnostic pathology, where CD45 staining helps differentiate hematopoietic malignancies from non-hematopoietic tumors. Automated image analysis improves accuracy and reproducibility. Unlike flow cytometry, which provides single-cell quantitative data, IHC offers spatial context, revealing CD45-positive cell distribution within tissues.

Diagnostic Applications In Hematology

CD45 is vital in hematological diagnostics, distinguishing leukocyte subsets and identifying malignancies. Since nearly all nucleated hematopoietic cells express CD45, its presence—or absence—guides blood disorder classification. Flow cytometry is the primary tool for assessing CD45 expression, differentiating blasts, lymphocytes, monocytes, and granulocytes based on fluorescence intensity and side scatter characteristics.

In acute leukemias, CD45 expression helps distinguish lymphoid from myeloid lineages. Acute lymphoblastic leukemia (ALL) blasts typically exhibit dim CD45 expression, while acute myeloid leukemia (AML) blasts show variable levels. These patterns assist in guiding diagnostics and immunophenotypic panel selection.

Beyond leukemia, CD45 aids in diagnosing lymphomas and other hematopoietic disorders. In non-Hodgkin’s lymphoma, aggressive forms often show reduced or absent CD45RO expression, a marker of mature T and B cells. Classical Hodgkin’s lymphoma is characterized by the absence of CD45 on Reed-Sternberg cells, differentiating it from other lymphoproliferative diseases.

IHC plays a crucial role in these evaluations, as CD45 staining helps distinguish hematologic malignancies from carcinomas or sarcomas, which lack CD45 expression. Integrating CD45 analysis with additional immunophenotypic markers enhances diagnostic accuracy, guiding treatment decisions and prognostic assessments in hematologic oncology.