T-cell apoptosis in inflammatory brain lesions: destruction of T cells does not depend on antigen recognition

Abstract

Elimination of inflammatory T cells by apoptosis appears to play an important role in the down-regulation of inflammation in the central nervous system. Here we report that apoptosis of T lymphocytes occurs to a similar extent in different models of autoimmune encephalomyelitis. Apoptosis is restricted to cells located in the neuroectodermal parenchyma, thereby leaving T cells present in the brain's connective tissue compartments unharmed. Death of T cells in the parenchyma does not depend on antigen presentation by resident microglial cells or astrocytes. Adoptive transfer experiments with T lymphocytes carrying a specific genetic marker revealed that in the central nervous system these cells are destroyed regardless of their antigen specificity or state of activation. Although many of both antigen-dependent and -independent mechanisms in the induction of T-cell apoptosis may act simultaneously, our results suggest that the nervous system harbors a specific, currently undefined, mechanism that effectively eliminates infiltrating T lymphocytes.

Figure 1.

a: Apoptosis in W3/13⁺ T cells. Apoptotic cells with a condensed nucleus (arrowhead) mostly are found singularly in the spinal cord parenchyma (MBP EAE, day 4; magnification, ×855). b: In some cases, apoptotic T cells are found in clusters of variable size (S100 EAE, day 4; ×855). c to e: Double staining for proliferation marker PCNA and W3/13. c shows a W3/13⁺ T cell (red, arrow) in close apposition to a PCNA⁺ nucleus (arrowhead) probably from a macrophage (MBP EAE, day 4; ×855). d and e: PCNA (black, arrowhead) and W3/13 (red) double-stained T cells. The homogenously stained nucleus with PCNA immunoreactivity against the nucleus (arrowheads) shows clear condensation, suggesting that these cells are undergoing apoptosis (MBP EAE, day 4; ×855). f: ISH for TK-tsA showing labeling of cells in the spinal cord of a TK-tsA transgenic rat with EAE (day 6) induced by TK-tsA MBP-specific T cells (×213). g: Higher magnification of f. All cells in the perivascular infiltrate show TK-tsA labeling in one or two distinct (brown) spots on the nucleus (×855). h to k: Transfer of TK-tsA MBP-specific T cells in normal Lewis rats. h: Low magnification of a Lewis rat with EAE induced by TK-tsA⁺ MBP-specific T cells on day 4 after transfer. High numbers of TK-tsA⁺ cells can be found around blood vessels and deeply infiltrated in the parenchyma (×336). i: Normal TK-tsA⁺ MBP cells are labeled with two clear distinct spots on the nucleus (arrow), whereas apoptotic cells (arrowhead) show a strong diffuse labeling over the entire nucleus (×855). j and k: ISH for TK-tsA showing two examples of apoptotic MBP-specific T cells in a Lewis rat EAE lesion that demonstrate the presence of apoptotic bodies (arrowheads) near the main part of the apoptotic cells (×855). l to n: ISH for TK-tsA. Transfer of Lewis MBP-specific T cells in TK-tsA rats (day 6). l: Low magnification of a longitudinal section of the spinal cord. High numbers of TK-tsA-labeled T cells are present in the meninges (m; ×266). m: TK-tsA-positive cells with the morphological appearance (diffuse staining over the entire nucleus) of apoptosis (arrowhead) can be found in between normal TK-tsA-labeled cells with one or two spots on the nucleus (×855). n: In addition to the TK-tsA-labeled apoptotic cells, TK-tsA-negative (MBP-specific) apoptotic cells (arrowhead) can also be found. In this case, the apoptotic cells lie in close apposition to a TK-tsA-labeled macrophage (×855). o and p: Transfer of activated TK-tsA OVA-specific T cells in a Lewis rat with EAE induced by Lewis MBP-specific T cells (day 4 after transfer). o: ISH for TK-tsA reveals the presence of many infiltrated OVA-specific T cells in the spinal cord (×266). p: ISH for TK-tsA in a higher magnification shows that besides normal TK-tsA⁺ OVA-specific T cells (two dots on the nucleus), apoptotic OVA-specific T cells can also be found (×855). q and r: Transfer of resting TK-tsA OVA-specific T cells in animals with EAE results in migration of low numbers of OVA-specific T cells in the spinal cord parenchyma (day 4 after transfer). Both normal OVA-specific T cells (q; ×855) and OVA-specific T cells with an apoptotic appearance (r; ×855) can be found.

Figure 2.

Infiltration and apoptosis of T cells in various models of EAE. a: percentage of T cells (±SEM) in the parenchyma and in meninges/perivascular space in models of EAE induced by antigen-specific T-cell lines against MBP (cell line C1; Lew MBP), MOG, MAG, and S100. In addition, EAE was induced by TK-tsA MBP-specific T cells in Lewis rats (TK-tsA MBP). b: percentages (±SEM) of apoptotic T cells in the parenchyma and in the meninges/perivascular space in these different models of EAE.

Figure 3.

Numbers of (apoptotic) T lymphocytes in parenchyma and meninges/perivascular space during the course of EAE. The average number (with SEM) of total T cells and apoptotic T cells was quantified in lumbar spinal cord cross sections on days 4, 6 and 9 of animals with MBP-T cell-induced EAE.