In situ tolerance within the central nervous system as a mechanism for preventing autoimmunity

Abstract

Multiple sclerosis (MS) is believed to be an autoimmune disease in which autoreactive T cells infiltrate the central nervous system (CNS). Animal models of MS have shown that CNS-specific T cells are present in the peripheral T cell repertoire of healthy mice and cause autoimmune disease only when they are activated by immunization. T cell entry into the CNS is thought to require some form of peripheral activation because the blood-brain barrier prohibits trafficking of this tissue by naive cells. We report here that naive T cells can traffic to the CNS without prior activation. Comparable numbers of T cells are found in the CNS of both healthy recombinase activating gene (Rag)(-/)- T cell receptor (TCR) transgenic mice and nontransgenic mice even when the transgenic TCR is specific for a CNS antigen. Transgenic T cells isolated from the CNS that are specific for non-CNS antigens are phenotypically naive and proliferate robustly to antigenic stimulation in vitro. Strikingly, transgenic T cells isolated from the CNS that are specific for myelin basic protein (MBP) are also primarily phenotypically naive but are unresponsive to antigenic stimulation in vitro. Mononuclear cells from the CNS of MBP TCR transgenic but not nontransgenic mice can suppress the response of peripheral MBP-specific T cells in vitro. These results indicate that naive MBP-specific T cells can traffic to the CNS but do not trigger autoimmunity because they undergo tolerance induction in situ.

Figure 1

Similar numbers of T cells are isolated from the CNS of MBP TCR transgenic mice and nontransgenic (Ntg) mice on both the Rag^−/− and Rag^+/+ background. CNS lymphocytes were collected as described in Materials and Methods and stained with anti-TCR monoclonal antibodies. Error bars represent one SD.

Figure 3

The number of CNS T cells displaying a naive phenotype significantly decreases as MBP TCR1 transgenic mice age. CNS (♦) and splenic (□) T cells were harvested from 21 nontransgenic and 13 MBP TCR1 transgenic mice of different ages and stained with anti-TCR, anti-CD44, and anti-CD45RB monoclonal antibodies. Percent naive represents the percentage of TCR⁺ cells that are neither CD44^hi nor CD45RBlo, as indicated in Fig. 2.

Figure 2

T cells in the CNS of TCR transgenic mice have a predominately naive phenotype. T cells were harvested from the CNS (solid lines) and spleen (dotted lines) as described in Materials and Methods, and stained with monoclonal antibodies specific for TCR, CD44, and CD45RB. The indicated percentage of naive CNS T cells represents the average percentage of TCR⁺ cells ± one SD that are neither CD44^hi nor CD45RB^lo (as indicated by the bars).

Figure 4

CNS T cells from MBP TCR1 and MBP TCR2 transgenic mice do not proliferate in response to antigen while MBP transgenic T cells recovered from CNS-draining cervical LNs (CLN) or nondraining inguinal LNs (ILN) proliferate well. (A) Flow cytometric analysis measuring the proliferation of pooled CNS T cells purified from spinal cords of 17 MBP TCR1 transgenic mice and LN T cells from the same animals. CNS and LN T cells were separately incubated with irradiated T cell–depleted wild-type splenic APCs with and without MBP Ac 1-11. Proliferating T cells are identified as BrdU⁺ cells as indicated by the bar. (B) Comparison of the proliferation of CNS and LN T cells from different TCR transgenic models in response to antigenic stimulation. Experiments were performed as described in panel A and the data represent an average of three experiments using MBP TCR1 transgenic mice and one experiment each pooling 15–17 MBP TCR2, DO11.10, and TEa TCR transgenic mice. For MBP TCR1 and TEa TCR transgenic mice, proliferation of V_α2⁺ CNS and LN T cells is shown. For MBP TCR2 and DO11.10 TCR transgenic mice, proliferation of TCR⁺ rather than V_α2⁺ T cells is shown. (C) The percentage of proliferating (BrdU⁺) V_α2⁺ T cells was determined for T cells harvested from the CLN and ILN (n = 2). Mice used for all of these experiments were 4–12 wk of age.

Figure 5

CNS mononuclear cells harvested from MBP TCR transgenic but not nontransgenic mice inhibit proliferation of MBP-specific peripheral LN cells in vitro. MBP TCR transgenic T cells isolated from peripheral LN cells were incubated with MBP Ac1-11 and irradiated APCs as described in the legend to Fig. 4 without (black bars) and with (hatched bars) additional CNS mononuclear cells. CNS mononuclear cells were isolated from 15–17 MBP TCR transgenic or nontransgenic mice. These data represent four experiments using CNS mononuclear cells from MBP TCR2 transgenic mice and three experiments using CNS mononuclear cells from nontransgenic mice. Mice used for these experiments were 4–12 wk of age. Tg, transgenic.