Human parathymic lymph node: morphological and functional significance

Abstract

Parathymic lymph nodes (PTLNs) have been identified in several species, but in humans they have been noted only once before in a study 90 years ago using fetal material. We now report their occurrence in children. Human PTLNs are small but distinctive lymphatic organs located on the surface of the thymus (or sometimes between the upper and lower lobes of the thymus) and covered with the thymic capsule. Histologically, the medullary cords of these lymph nodes were found to be thin, with only small numbers of plasma cells. In addition, they had a well-developed paracortical area rich with high endothelial venules (HEV), but a thin cortex, including only a few undeveloped follicles. Flow cytometric analysis of PTLNs revealed that the ratios of T:B cells (14.6+/-9.3) and of CD4+:CD8+ T cells (4.9+/-1.4) in PTLNs were much higher than in other peripheral lymphoid tissues and in peripheral blood. Because of these characteristics of the human PTLNs, we propose that the human PTLNs might influence the functional differentiation of T cells.

Figure 1

Thymus and PTLNs obtained from a 3-month-old female autopsy case. (a) Macroscopic view of the thymus and the PTLNs (arrows). The PTLNs are observed on the surface of the apex of the cervical thymus or between the cervical and thoracic lobes. (b) Microscopic view of the PTLN of the cervical thymus (H&E stain). The PTLN is covered by the common capsule of the thymus. In this node, there is hardly any medullar cord to be found ×30. (c) High magnification view of a part of the PTLN as framed in (b). A small follicle in the subcapsular area and many HEVs in paracortical area are found; ×150.

Figure 2

Histological findings of the PTLN and mesenteric lymph nodes (H&E stain). Representative figures using both the PTLN and the mesenteric lymph nodes from the same donor (20-month-old female autopsy case) are shown. (a) A PTLN; this node possessed a thin cortex with a small number of follicles; a large part of this node is dominated by the paracortical area; ×30. (b) A small mesenteric lymph node (×30), and (c) a large mesenteric lymph node (×15). The cortical parenchyma of these nodes consisted of an outer layer containing a number of large germinal centres while the medullary cords were well-developed.

Figure 3

Immunostaining of frozen sections of PTLN obtained from a 16-day-old male (undergoing open cardiac surgery). (a) Stained with anti-CD3 antibody; many of the PTLN cells are positive; ×150. (b) Stained with anti-CD20 antibody; the subcapsular area and a small primary follicle are stained; ×150. (c) High magnification view of HEV stained with anti-CD3 antibody. Adhesion between CD3-positive cells and high endothelial cells is found; ×450. (d) High magnification view of HEV stained with anti-CD20 antibody. Adhesion between CD20-positive cells and high endothelial cells is also found. CD20-positive cells are also present generally in the perivascular area; ×450. Arrowheads show the HEVs in the PTLN.

Figure 4

Two-colour immunofluorescence analysis of PTLN cells. Analysis of surface CD4/CD8 expression (a) and CD3/CD19 expression (b) with the percentage in each quadrant.

Figure 5

Three-colour immunofluorescence analysis of the expressions of CD3, CD45RA, CD45RO and IL-2Rβ in PTLN cells subsets defined by CD4 and CD8 surface expression (see to Fig. 4a). Cells are gated into CD4⁻ CD8⁻, CD4⁺ CD8⁻ and CD4⁻ CD8⁺ subsets, and the expressions of those surface antigens in each subset are shown as histograms.

Figure 6

Three-colour immunofluorescence analysis of separated CD3⁺ CD4⁺ CD8⁻ SP cells and CD3⁺CD4⁻ CD8⁻ double-negative cells from whole PTLN lymphocytes using a depletion technique with antibody coupled to paramagnetic beads. The purity of these two populations was 99%. The expressions of CD45RA, CD45RO and IL-2Rβ of separated single-positive and double-negative cells are shown as histograms.

Figure 7

Two-colour immunofluorescence analysis of the expression of TCR Vα24 and CD3 in PTLN cells. (a) The gated cells are TCR Vα24-positive, and the percentage is about 1·8%. (b) Isotypic control IgG1 was used instead of anti-TCR Vα24 antibody.