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101.
Mice lacking MHC class II molecules 总被引:47,自引:0,他引:47
We have produced mice that lack major histocompatibility complex class II antigens, permitting us to evaluate the role of these molecules in diverse aspects of T and B cell differentiation. The mutant mice show near-complete elimination of CD4+ T lymphocytes from the spleen and lymph nodes; the few remaining CD4-positive cells are preferentially localized to B cell follicles. Surprisingly, substantial numbers of CD4 single-positive cells reside in the thymus; however, these are not mature thymocytes as we currently recognize them. B lymphocytes occur in normal numbers and are capable of terminal differentiation to plasma cells. Nevertheless, several aberrations in the B cell compartment are demonstrable: a lack of germinal centers, fewer IgM+IgD+ cells in certain individuals, reduced production of serum IgG1, and complete inability to respond to T-dependent antigens. In short, the class II-negative mice have confirmed some old ideas about lymphocyte differentiation, but have provided some surprises. 相似文献
102.
G A Mathis P A Wyss E G Schuetz R P Hughey A E Sirica 《Journal of cellular physiology》1991,146(2):234-241
Freshly isolated hepatocytes from normal adult rat liver do not express measurable gamma-glutamyl transpeptidase (GGT) mRNA in contrast to the significant GGT mRNA levels expressed by normal adult rat kidney and hyperplastic bile ductular tissue from bile duct-ligated rats. However, the induction of GGT activity in rat hepatocytes by two-thirds hepatectomy was accompanied by the appearance of a high level of GGT mRNA. We are now able to demonstrate that normal adult rat hepatocytes express 5 protein bands which cross-react with 2 different anti-rat kidney GGT antisera. The apparent molecular weights were 26.9, 58.0, 63.9, 73.5, and 83.4 kDa, respectively. Expression of the 26.9- and 58.0-kDa proteins strikingly parallels the pattern of induction of GGT enzymatic activity. This suggests that these 2 proteins correspond to the active dimeric enzyme previously described in kidney and neoplastic hepatocellular tissue. In normal hepatocytes, the 73.5-kDa protein represents 50% of the total GGT-immunoreactive protein, in contrast to kidney, where this band contains less than 4% of the GGT protein. The kinetics of expression of the 73.5-kDa protein upon induction of GGT activity in hepatocytes, as well as in culture turnover studies, suggests that this protein is a precursor form of the active enzyme, such as the described 78/79-kDa single-chain glycoprotein propeptide of GGT. It appears that in normal hepatocytes, this precursor is not processed to the same extent as in kidney or in hyperplastic bile ductular tissue. 相似文献
103.
104.
Incubation of spinach thylakoids with HgCl2 selectively destroys Fe–S center B (FB). The function of electron acceptors in FB-less PS I particles was studied by following the decay kinetics of P700+ at room temperature after multiple flash excitation in the absence of a terminal electron acceptor. In untreated particles, the decay kinetics of the signal after the first and the second flashes were very similar (t
1/22.5 ms), and were principally determined by the concentration of the artificial electron donor added. The decay after the third flash was fast (t
1/20.25 ms). In FB-less particles, although the decay after the first flash was slow, fast decay was observed already after the second flash. We conclude that in FB-less particles, electron transfer can proceed normally at room temperature from FX to FA and that the charge recombination between P700+ and FX
-/A1
- predominated after the second excitation. The rate of this recombination process is not significantly affected by the destruction of FB. Even in the presence of 60% glycerol, FB-less particles can transfer electrons to FA at room temperature as efficiently as untreated particles.Abbreviations DCIP
2, 6-dichlorophenol indophenol
- FA, FB, FX
iron-sulfur center A, B and X, respectively
- PMS
phenazine methosulfate 相似文献
105.
Incubation of PS II membranes with herbicides results in changes in EPR signals arising from reaction centre components. Dinoseb, a phenolic herbicide which binds to the reaction centre polypeptide, changes the width and form of the EPR signal arising from photoreduced Q?AFe. o-Phenanthroline slightly broadens the Q?AFe signal. These effects are attributed to changes in the interaction between the semi-quinone and the iron. DCMU, which binds to the 32 kDa protein, has virtually no effect on the width of the Q?AFe signal but does give rise to an increase in its amplitude. This could result from a change in redox state of an interacting component. Herbicide effects can also be seen when Q?AFe is chemically reduced and these seen to be reflected by changes in splitting and amplitude of the split pheophytin? signal. Dinoseb also results in the loss of ‘Signal II dark’, the conversion of reduced high-potential cytochrome b559 to its oxidized low-potential form and the presence of transiently photooxidized carotenoid after a flash at 25°C; these effects indicate that dinoseb may also act as an ADRY reagent. 相似文献
106.
Photosystem I particles devoid of the secondary electron acceptor A1 were studied by nanosecond flash absorption. The primary radical pair (P-700+, A0
-) decays with a half-time of 35 ns. The difference spectrum was measured (400–870 nm). After subtraction of the P-700+/P-700 difference spectrum, the A0
-/A0 was obtained. It includes bleachings centered at 690 and 430 nm, and broad positive bands in the near infra-red and the blue-green. This spectrum is consistent with A0 being chlorophyll a absorbing at 690 nm. 相似文献
107.
Flash-induced absorption changes were studied on different timescales (nanosecond to millisecond) and at different temperatures (10 to 278 K) in highly enriched spinach PS I particles lacking vitamin K-1 and in which the electron transfer from the primary acceptor to the secondary acceptors was blocked. At all temperatures, the initial absorption change at 820 nm was followed by a fast decay (t1/2 ≈ 47 ns at 278 K and ≈ 82 ns at 10 K) which is attributed to the decay of the primary radical pair (P-700+-A−0). A slower phase of absorption decay is attributed to the P-700 triplet state, which was formed as a result of the biradical recombination, with a yield of about 30% at 278 K and about 75% at 10 K. Under air, the 3P-700 state decayed with a t1/2 of about 50 μs at 278 K, whereas in the absence of oxygen it decayed with t1/2 ≈ 560 μs. At 278 K, this yield was shown to depend on the presence of a magnetic field, with a maximum around 60 G. The 3P-700 decay halftime was nearly independent of temperature in the absence of oxygen (t1/2 ≈ 1 ms at 10 K). The implications for the mechanisms involved in this decay are discussed. Addition of vitamin K-1 to these particles resulted in a decrease in the amplitude of the fast submicrosecond decay and a concomitant increase in the amplitude of a slow phase, indicating an efficient transfer from A−0 to vitamin K-1. However, most functional properties of the acceptor A1 were not reconstituted under these conditions. 相似文献
108.
On the basis of absorption and circular dichroism (CD) spectral measurements, we conclude that the photoreduction of protochlorophyllide to chlorophyllide in homogenates of etiolated bean seedlings (Phaseolus vulgaris L.) involves two light steps in series. Before illumination, the active protochlorophyllide occurs in a dimeric form in the holochrome protein. The initial light reaction converts one of the protochlorophyllide molecules and forms a chlorophyllide-protochlorophyllide holochrome intermediate with a weak, characteristic CD spectrum. The second light reaction subsequently converts the second protochlorophyllide in a less efficient reaction that is temperature dependent. This produces a chlorophyllide holochrome which exhibits a strong double CD characteristic of dimers and which is stable below 1°C. At higher temperatures this dimeric chlorophyllide transforms in the dark to a monomeric form with low CD amplitude. Sucrose at high concentrations (2 M) alters the chlorophyllide holochrome CD spectrum and prevents the final dark dissociation step. Analysis of the photochemical kinetics confirms the occurrence of the two-step photoreduction and supports the stoichiometry of two (proto)chlorophyllides per holochrome protein. 相似文献
109.
Antigen/MHC-specific T cells are preferentially exported from the thymus in the presence of their MHC ligand 总被引:35,自引:0,他引:35
Transgenic mice expressing a T cell receptor heterodimer specific for a fragment of pigeon cytochrome c plus an MHC class II molecule (I-Ek) have been made. We find that H-2k alpha beta transgenic mice have an overall increase in the number of T cells and express a 10-fold higher fraction of cytochrome c-reactive cells than H-2b mice. Surface staining of thymocytes indicates that in H-2b mice, T cell development is arrested at an intermediate stage of differentiation (CD4+8+, CD310). Analyses of mice carrying these T cell receptor genes and MHC class II I-E alpha constructs indicate that his developmental block can be reversed in H-2b mice by I-E expression on cortical epithelial cells of the thymus. These data suggest that a direct T cell receptor-MHC interaction occurs in the thymus in the absence of nominal antigen and results in the enhanced export of T cells, consistent with the concept of "positive selection". 相似文献
110.
Arjan D van Zuilen Ingeborgh van der Tweel Peter J Blankestijn Michiel L Bots Marjolijn van Buren Marc AGJ ten Dam Karin AH Kaasjager Peter JG van de Ven Gerald Vervoort Louis-Jean Vleming Gerry Ligtenberg Jack FM Wetzels 《Trials》2006,7(1):1-9