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Sponsored and reviewed by ICCS Quality and Standards Committee

Title: Expression of CD5 on Normal Hematolymphoid Cells

Written by: Weina Chen and “Buddy” Frank Fuda

Date: Jan 25, 2019

______________________________________________________________________ INTRODUCTION CD5 was one of the first surface markers used to identify T-cells. It is a transmembrane glycoprotein expressed on both immature and mature T-cells. Functionally, it is a co-receptor that can either inhibit or promote T-cell activation by modulating the T-cell receptor (TCR)/peptide major histocompatibility (pMHC) signaling pathway. Co-inhibitory or co-stimulatory effects depend upon the maturation state and the location of the T-lineage cell being activated1. Clinical flow cytometric analysis identifies subsets of T-lineage cells that show different intensities of CD5 expression. While CD5 is generally considered a T-lineage associated marker, its expression extends to other lineage cells, such as subsets of B-cells, NK-cells, and dendritic cells. We will briefly describe normal CD5 expression patterns on various hematolymphoid cells and briefly comment on its diagnostic implications. CD5 expression on T-lineage cells

A. Thymocytes: Thymocytes are immature T-lineage cells that undergo defined stages of maturation. All thymocytes express CD5 and show increased intensity of expression at later stages of maturation. In this example, thymocytes from a thymic specimen are gated and colored green. The sequence of maturation is designated by the black arrows.

B. TCR α/β Mature T-lymphocytes: Mature TCR α/β CD4(+) helper T-cells typically show uniform bright intensity of CD5 expression. Mature TCR α/β CD8(+) cytotoxic T-cells commonly show slightly dimmer CD5 expression compared to TCR α/β CD4(+) helper T-cells and can have uniform or variable CD5 expression intensity. Examples are shown on these plots from a peripheral blood specimen and from a lymph node specimen. Please note that variability of CD5 expression on mature TCR α/β CD8(+) cytotoxic T-cells is not tissue specific but rather can be seen in any tissue. Mature T-cells are colored green.

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C. TCR γ/δ Mature T-lymphocytes: Mature TCR γ/δ T-cells typically show variable CD5 expression from dim to negative. In the ungated data analysis from a bone marrow specimen shown here, TCR γ/δ T-cells are colored blue and TCR α/β T-cells are colored green for comparison. Notice that the typical staining pattern for TCR γ/δ T-cells is slightly brighter for surface CD3 and dimmer for CD5 when compared to TCR α/β T-cells. (Please note that normal TCR γ/δ T-cells occasionally show increased variability for intensity of CD3 expression from positive to bright positive).

Peripheral Blood Lymph Node

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D. Aberrant expression of CD5 on Mature T-cells: Familiarity with typical CD5 expression patterns in T-cell subsets is often useful in helping to differentiate normal from neoplastic T-cells. In the plot here, T-lineage cells from a body fluid sample are represented in an ungated analysis. The TCR α/β T-cells are colored green and TCR γ/δ T-cells are colored blue. Unlike the previous plot, the TCR γ/δ T-cells in this case completely lack expression of CD5 and show unusually low expression of CD3. These aberrant features are often seen in TCR γ/δ T-cell lymphoma, which was the final diagnosis.

CD5 expression on B-lineage cells

Immunology literature describes 2 broad populations of B-cells in mice, referred to as B1 and B2 cells. The B1 cell population contains a subset that expresses CD5, known as B1-a cells. While the existence of an exact human counterpart to murine B1 cells remains controversial,2,3,4 clinical flow cytometry clearly identifies expression of CD5 on subsets of human B-cells in both immature and mature B-cell compartments.5 These CD5(+) B-cell subsets can be identified in various body tissues and can account for relatively high proportions of the total B-cells, especially under certain reactive conditions. Recognizing these subsets as normal is particularly important, as CD5 expression is a defining feature of some B-cell neoplasms, such as chronic lymphocytic leukemia/lymphoma (CLL/SLL) and mantle cell lymphoma. Differentiating normal from neoplastic CD5(+) B-lineage cells can typically be accomplished by ensuring that the CD5(+) subset is polytypic for surface immunoglobulin light chain and that it lacks immunophenotypic aberrancy. A few examples of CD5(+) B-lineage cells in different types of samples (bone marrow, peripheral blood and tissue) are presented here.

A. Bone Marrow: B-cell maturation occurs in bone marrow. Immature B-cells, known as hematogones, undergo a consistent and reproducible spectrum of sequential antigen expression to become mature naïve B-lymphocytes. These maturation steps can be broken down into stages 1 through 36,7

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The following is an ungated B-cell analysis from a normal bone marrow specimen. In the first row, all B-lineage cells are colored blue with red arrows showing the maturation pattern for hematogones. The 3 stages of hematogone maturation are noted on the CD10 vs CD20 plot. In the CD5 vs. CD19 plot in the 2nd row, it is evident that a proportion of B-lineage cells expresses CD5 (highlighted green). These CD5(+) B cells comprise mostly stage 3 hematogones and mature B-cells.5 Highlighting the CD5(+) hematogones black and the CD5(+) mature B-cells red (far right plot of 2nd row) allows identification of their polytypic nature (3rd row).

Please note that the intensity of CD20 and surface immunoglobulin light chain (sIg) on the CD5(+) B cells is similar to the remaining mature B cells. Both populations thus show expected normal immunophenotypic features, which nearly eliminates the possibility of a biclonal CLL/SLL or a CLL/SLL with a subset that has presumably undergone an Ig light chain switch. Such populations would artificially appear “polytypic” but would typically demonstrate aberrantly dim CD20 and dim sIg expression. See the example of a CD5(+) CLL/SLL showing a pseudo “polytypic” pattern of surface Ig expression in the second set of plots under the heading lymph node.

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If the polytypic CD5(+) B-cell subset does not show overt aberrancy (e.g., abnormal intensity of surface immunoglobulin light chain, CD20 or CD5 expression) but accounts for a relatively high proportion of total B-cells (e.g., greater than 20%), the pathologist reviewing a concurrent morphologic specimen should be informed. If clinical or morphologic suspicion is high for lymphoma/leukemia, additional studies could be considered. For example, a portion of the specimen could be sent for fluorescent in situ hybridization (FISH) for t(11;14)(q13;q32) to exclude the rare event of a biclonal mantle cell lymphoma.

B. Peripheral Blood: Normal CD5(+) B-cells can also be identified in peripheral blood. If hematogones are present, they are typically limited to stage 3.8

In this example, the B-lineage cells are gated for easier visualization. This case contains both CD5(+) stage 3 hematogones and mature CD5(+) B-lymphocytes (colored together as blue) that exhibit polytypic light chain expression. The mature CD5(-) B-cells are colored green for comparison. Please note that comparable to the normal bone marrow, the intensity of CD20 and surface Ig light chain expression is similar between the peripheral blood CD5(+) B-cells and mature CD5(-) B-cells.

C. Lymph Node In lymph node, CD5(+) B-lymphocytes predominantly inhabit the CD10(-) compartment.

Although in rare instances, CD10(+)/CD5(+) stage 3 hematogones can also be seen.

A B-cell analysis performed on ungated data from a normal lymph node is presented in the 1st row of plots below. The typical pattern of CD5 expression on normal B-lymphocytes is illustrated. With regard to expression of CD5, notice the seamless transition from CD5(-) B-cells (blue) to CD5(+) B-cells (red). In addition, notice that the intensity of CD5 expression remains dim relative to the normal T-cells (green). The CD5(+) B-cells show normal intensity of both CD20 and sIg light chain.

Contrast these features to the neoplastic CD5(+) B-cells in the 2nd row. Here, rather than a seamless transition from CD5(-) to CD5(+) B-cells, the two populations are more distinctly separated [see CD5(+) neoplastic B-cells colored red and normal CD5(-) B-cells colored blue]. In addition, the intensity of CD5 expression on the neoplastic B-cells reaches that of the normal T-cells (green), although this is not a consistent finding of neoplastic CD5(+) B-cells.

While this CD5(+) neoplastic B-cell population shows a pseudo “polytypic” sIg staining pattern in this example, these cells show aberrantly dim expression of CD20 and aberrantly dim expression of sIg, which are typical aberrant features of CLL/SLL. This case is an example of a biclonal CLL/SLL (characterized by strong CD5, dim CD20 and dim sIg), as confirmed by molecular clonality studies (not shown) and clinical features. For a detailed discussion of biclonal CLL see ICCS Q&S standards module #9 by Maria Arroz (https://www.cytometry.org/web/quality.php).

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D. Other tissues CD5(+) B-cells are also common in the spleen and body cavity fluids.10,11

The following table (Table 1) shows immunophenotypic features helpful in differentiating normal CD5(+) B-cells from chronic lymphocytic leukemia/small lymphocytic lymphoma and mantle cell lymphoma. Table1. CD5 expression in normal B cells and B-cell lymphomas Markers

Normal CD5(+) B-cells CD5(+) CLL/SLL CD5(+) Mantle Cell Lymphoma

CD5 Seamless transition from CD5(-) cells to CD5 dim (+) cells

- Often distinct from normal CD5(-) B-cells - Often moderate to bright expression of CD5

- Often distinct from normal CD5(-) B-cells - Often moderate to bright expression of CD5

CD20 Expression equivalent to CD5(-) B-cells

Dim expression Expression equivalent to normal B-cells

Surface Ig Polytypic and Expression equivalent to CD5(-) B-cells

-Typically monotypic -Dim expression

-Typically monotypic - Expression equivalent to normal B-cells

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CD5 expression on NK lymphocytes and dendritic cells

Subsets of NK lymphocytes, myeloid dendritic cells, and plasmacytoid dendritic cells can also express CD5;12,13,14 however, if present these cells typically represent a very low proportion of the total cell populations. Therefore, they are unlikely to be mistaken as neoplastic subsets in de novo cases. They may present problems when assessing for minimal residual T-lymphoblastic leukemia or T-cell lymphoma, as both NK cells and dendritic cell subsets express several other T-lineage associated markers. NK cells

Let’s look at an example of NK cells from an ungated peripheral blood analysis. In the plot presented here, T-cells are colored green. Note that there is a subset of uncolored, CD3(-)/CD5(+) cells (see arrow/circle) that accounts for 3.1% of total cells. This population may represent a neoplastic population, a normal B-lineage population, a normal NK lymphocyte population, or a normal dendritic cell population. B-cells from the same analysis are highlighted red in the following plot. It is clear that there is a CD5(+) subset (see arrow/circle) of polytypic B-cells (sIg expression not shown); however, this subset accounts for only 0.23% of total cells. This leaves 2.87% CD5(+) cells unaccounted for by T and B-cells (3.1% non-T-cell CD5(+) cells minus 0.23% CD5(+) B-cells). Therefore, there must be another population of non-B or T-cells that expresses CD5.

T-cells

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Further analysis of ungated data in the plots below shows that these cells are a subset of CD5(+) NK lymphocytes (colored blue below). T-cells are colored green for comparison. Here, there are several subsets of NK cells (all colored blue) that show variable expression of NK-associated markers CD2, CD7, CD8, and CD56 but lack expression of the T-lineage specific marker surface CD3. The CD5(+) NK-cell subsets are circled.

B-cells

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Myeloid Cells

Lastly, let’s take a look at an ungated analysis of a pleural fluid specimen containing a population of myeloid cells with a subset that expresses CD5. These cells express the myeloid markers CD13, CD33 and CD64partial and the antigen presenting cell marker HLA-DR. They mostly lack expression of monocytic associated markers CD14 and CD36 (not shown) but have bright expression of CD11c. Overall, these cells have an immunophenotype compatible with myeloid dendritic cells. Additional dendritic cell markers were not available for assessment. It is important to note that in most samples subsets of CD5(+) myeloid cells are rare events that go unrecognized. Occasionally, they will be expanded into a readily identifiable population, as in this case.

SUMMARY In summary, CD5 is a rather promiscuous marker not limited to expression on normal T-lineage cells. Its expression on subsets of immature and mature B-lineage cells is readily identified in many samples from various tissues. In contrast, its expression on NK lymphocytes and dendritic cells is much less commonly seen but can become apparent in certain reactive conditions and in high sensitivity analysis for minimal residual disease detection. Knowledge of all cell types that normally express CD5 and its normal expression pattern (such as dim intensity) is important to prevent an unnecessary and protracted evaluation and to prevent an erroneous designation of a reactive population as a B-cell or T-cell lymphoma.

Specifically reporting CD5 expression on these normal hematolymphoid subsets is not necessary; however, it is important to recognize them as a normal finding

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