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Intraepithelial versus thymic lymphocyte selection
U P D AT E I M M U N O L O G Y TO D AY
Current literature TCR phosphorylation determines T-cell activation Neumeister Kersh, E., Shaw, A.S. and Allen, P.M. (1998) Fidelity of T cell activation through multistep T cell receptor phosphorylation Science 281, 572–575 The T-cell receptor (TCR) comprises an ␣ heterodimer and the CD3 complex: this consists of the ␥, ␦, ⑀ and polypeptide chains. The ␥, ␦ and ⑀ chains each contain one immunoreceptor tyrosine-based activation motif (ITAM) which, when phosphorylated, can interact with kinases and initiate T-cell activation. The chain, however, contains three ITAMs, thought to amplify signal transduction from the TCR but, as investi-
Intraepithelial versus thymic lymphocyte selection Cruz, D., Sydora, B.C., Hetzel, K. et al. (1998) An opposite pattern of selection of a single T cell antigen receptor in the thymus and among intraepithelial lymphocytes J. Exp. Med. 188, 255–265 Extrathymic differentiation of intestinal intraepithelial lymphocytes (IELs) is likely, however, the use of T-cell receptor (TCR) transgenic animals to investigate IEL selection has been hampered by endogenously rearranged and expressed TCR molecules in these animals. Here, Cruz et al. tackle the question by using RAG-2⫺/⫺ knockout mice to express an
Inflammation not imitation in virally induced diabetes Horwitz, M.S., Bradley, L.M., Harbertson, J. et al. (1999) Diabetes induced by Coxsackie virus: Initiation by bystander damage and not molecular mimicry Nat. Med. 4, 781–785 Virally induced autoimmunity is ascribed either to the effects of bystander T-cell activation or to molecular mimicry by viral antigens of self components. Insulin dependent diabetes mellitus (IDDM) in humans is often associated with Coxsackie B4 (CB4) infection. However, although CB4 has strong tropism for the pancreas, predictive of inflammation, its P2-C protein also appears to mimic the glutamic acid decarboxylase 65
gated here by Neumeister Kersh et al., its purpose could be to qualitatively evaluate ligands of the ␣ TCR. The chain has six potential phosphorylation sites and exists in two phosphoforms of 21 and 23 kDa in resting and activated T cells, respectively. Six antisera specific for each of the phosphotyrosines were used to study the molecular composition of , revealing that the p21 form has two permanent phosphotyrosines in resting T cells. By contrast, on stimulation of the T-cell clone the 23 kDa phosphoform appeared, which was recognized by all six antisera demonstrating that all six tyrosines in become phosphorylated on full Tcell activation. By mutating each of the six Tyr sites to Phe and subsequent probing with
each of the antisera, the phosphorylation of four sites was revealed to be interdependent, indicating an ordered progression from basal to full phosphorylation. The effects of the nature of the TCR ligand on phosphorylation were examined using altered peptide ligands (APLs). These APLs bound to the MHC with similar affinities but induced T-cell function to varying degrees. phosphorylation was initiated and proceeded substantially for the strongest APL, but was not completed for any of the suboptimal ligands. This study suggests that ordered phosphorylation steps of the chain act as thresholds that serve to convert the various TCR–ligand interactions into different biological responses.
H-Y/H-2Db-specific TCR transgene, thus precluding endogenous TCR rearrangement. The male-specific antigen H-Y is absent in females, thus allowing TCR selection to be compared between males and females. Negative selection of the H-Y TCR in males results in a severely hypocellular thymus, compared with females. However, in the small intestine the situation is reversed, with male animals having up to 90-fold more IELs. Significantly, the IELs that are present in female small intestine are mostly CD3⫺ with some TCR⫹ CD4⫺CD8⫺ cells. Selection and expansion of TCR⫹ CD8␣⫹ IELs does not occur in females. The male IELs are predominantly TCR⫹ CD8␣␣⫹: some of these also express
low levels of CD8, possibly as CD8␣ homodimers. Importantly, TCR⫹ CD8␣␣⫹ IELs are not anergic, thus have not simply undergone negative selection without apoptosis. Clearly, cells in the thymus and small intestine have been differently selected, although this is not confirmation of extrathymic selection. The relative lack of IELs in the small intestine of female H-Y/RAG-2⫺/⫺ mice shows that IEL development does not follow from positive thymic selection. In fact, the data from their male counterparts suggest that IELs are independent of effective thymic selection of CD8⫹ TCR⫹ cells, and that CD8␣␣ homodimer expression may be the key to IEL development.
(GAD65) islet antigen. Here, Horwitz et al. show that CB4-induced IDDM in nonobese diabetic (NOD) mice is due to inflammation followed by bystander T-cell activation to liberated antigens. The study compared CB4-induced IDDM in spontaneously diabetic NOD mice, which respond to GAD65 in the context of the I-Ag7 MHC class II molecule; in B10.H2g7 mice which have I-Ag7 but do not develop IDDM; and in BDC2.5 mice which are transgenic for a diabetogenic TCR that does not recognize GAD65. CB4 infection of pre-diabetic NOD mice did not accelerate the onset of IDDM, or cause islet destruction, despite induction of a severe pancreatitis. The same was true of CB4 infection of
B10.H2g7 mice: in neither B10.H2g7 nor NOD mice were GAD65-specific T-cell responses elevated or accelerated. In BDC2.5 mice, however, which carry a GAD65-unrelated diabetogenic TCR, CB4 infection rapidly leads to islet destruction and IDDM. The lack of CB4-induced IDDM in prediabetic NOD mice, compared with BDC2.5 animals is strongly suggestive of bystander activation following initial inflammation. The BDC2.5 T-cell repertoire is abnormally skewed to an, as yet undefined, islet antigen which is made available following CB4-induced inflammation. A key role for molecular mimicry would have accelerated onset of diabetes in CB4-infected NOD animals.
SEPTEMBER Vo l . 1 9
No.9
1 9 9 8 383
Current literature TCR phosphorylation determines T-cell activation Neumeister Kersh, E., Shaw, A.S. and Allen, P.M. (1998) Fidelity of T cell activation through multistep T cell receptor phosphorylation Science 281, 572–575 The T-cell receptor (TCR) comprises an ␣ heterodimer and the CD3 complex: this consists of the ␥, ␦, ⑀ and polypeptide chains. The ␥, ␦ and ⑀ chains each contain one immunoreceptor tyrosine-based activation motif (ITAM) which, when phosphorylated, can interact with kinases and initiate T-cell activation. The chain, however, contains three ITAMs, thought to amplify signal transduction from the TCR but, as investi-
Intraepithelial versus thymic lymphocyte selection Cruz, D., Sydora, B.C., Hetzel, K. et al. (1998) An opposite pattern of selection of a single T cell antigen receptor in the thymus and among intraepithelial lymphocytes J. Exp. Med. 188, 255–265 Extrathymic differentiation of intestinal intraepithelial lymphocytes (IELs) is likely, however, the use of T-cell receptor (TCR) transgenic animals to investigate IEL selection has been hampered by endogenously rearranged and expressed TCR molecules in these animals. Here, Cruz et al. tackle the question by using RAG-2⫺/⫺ knockout mice to express an
Inflammation not imitation in virally induced diabetes Horwitz, M.S., Bradley, L.M., Harbertson, J. et al. (1999) Diabetes induced by Coxsackie virus: Initiation by bystander damage and not molecular mimicry Nat. Med. 4, 781–785 Virally induced autoimmunity is ascribed either to the effects of bystander T-cell activation or to molecular mimicry by viral antigens of self components. Insulin dependent diabetes mellitus (IDDM) in humans is often associated with Coxsackie B4 (CB4) infection. However, although CB4 has strong tropism for the pancreas, predictive of inflammation, its P2-C protein also appears to mimic the glutamic acid decarboxylase 65
gated here by Neumeister Kersh et al., its purpose could be to qualitatively evaluate ligands of the ␣ TCR. The chain has six potential phosphorylation sites and exists in two phosphoforms of 21 and 23 kDa in resting and activated T cells, respectively. Six antisera specific for each of the phosphotyrosines were used to study the molecular composition of , revealing that the p21 form has two permanent phosphotyrosines in resting T cells. By contrast, on stimulation of the T-cell clone the 23 kDa phosphoform appeared, which was recognized by all six antisera demonstrating that all six tyrosines in become phosphorylated on full Tcell activation. By mutating each of the six Tyr sites to Phe and subsequent probing with
each of the antisera, the phosphorylation of four sites was revealed to be interdependent, indicating an ordered progression from basal to full phosphorylation. The effects of the nature of the TCR ligand on phosphorylation were examined using altered peptide ligands (APLs). These APLs bound to the MHC with similar affinities but induced T-cell function to varying degrees. phosphorylation was initiated and proceeded substantially for the strongest APL, but was not completed for any of the suboptimal ligands. This study suggests that ordered phosphorylation steps of the chain act as thresholds that serve to convert the various TCR–ligand interactions into different biological responses.
H-Y/H-2Db-specific TCR transgene, thus precluding endogenous TCR rearrangement. The male-specific antigen H-Y is absent in females, thus allowing TCR selection to be compared between males and females. Negative selection of the H-Y TCR in males results in a severely hypocellular thymus, compared with females. However, in the small intestine the situation is reversed, with male animals having up to 90-fold more IELs. Significantly, the IELs that are present in female small intestine are mostly CD3⫺ with some TCR⫹ CD4⫺CD8⫺ cells. Selection and expansion of TCR⫹ CD8␣⫹ IELs does not occur in females. The male IELs are predominantly TCR⫹ CD8␣␣⫹: some of these also express
low levels of CD8, possibly as CD8␣ homodimers. Importantly, TCR⫹ CD8␣␣⫹ IELs are not anergic, thus have not simply undergone negative selection without apoptosis. Clearly, cells in the thymus and small intestine have been differently selected, although this is not confirmation of extrathymic selection. The relative lack of IELs in the small intestine of female H-Y/RAG-2⫺/⫺ mice shows that IEL development does not follow from positive thymic selection. In fact, the data from their male counterparts suggest that IELs are independent of effective thymic selection of CD8⫹ TCR⫹ cells, and that CD8␣␣ homodimer expression may be the key to IEL development.
(GAD65) islet antigen. Here, Horwitz et al. show that CB4-induced IDDM in nonobese diabetic (NOD) mice is due to inflammation followed by bystander T-cell activation to liberated antigens. The study compared CB4-induced IDDM in spontaneously diabetic NOD mice, which respond to GAD65 in the context of the I-Ag7 MHC class II molecule; in B10.H2g7 mice which have I-Ag7 but do not develop IDDM; and in BDC2.5 mice which are transgenic for a diabetogenic TCR that does not recognize GAD65. CB4 infection of pre-diabetic NOD mice did not accelerate the onset of IDDM, or cause islet destruction, despite induction of a severe pancreatitis. The same was true of CB4 infection of
B10.H2g7 mice: in neither B10.H2g7 nor NOD mice were GAD65-specific T-cell responses elevated or accelerated. In BDC2.5 mice, however, which carry a GAD65-unrelated diabetogenic TCR, CB4 infection rapidly leads to islet destruction and IDDM. The lack of CB4-induced IDDM in prediabetic NOD mice, compared with BDC2.5 animals is strongly suggestive of bystander activation following initial inflammation. The BDC2.5 T-cell repertoire is abnormally skewed to an, as yet undefined, islet antigen which is made available following CB4-induced inflammation. A key role for molecular mimicry would have accelerated onset of diabetes in CB4-infected NOD animals.
SEPTEMBER Vo l . 1 9
No.9
1 9 9 8 383