Long-term ventilation in children with severe central nervous system impairment

Absent from the list of indications for long-term ventilation (LTV) is its use for children with severe central nervous system impairment, including those with severe mental retardation or in a permanent vegetative state. Over a two year period, we evaluated eight children with severe CNS dysfunction for whom long-term ventilation was being contemplated. Of these eight patients, three were in a permanent vegetative state and the remainder were severely neurologically impaired, with minimal cognition. The following recommendations were developed: (1) LTV for patients in a permanent vegetative state is inappropriate. (2) In a patient with severe neurologic disease, the process of informed consent must be viewed as dynamic; once the patient's condition is diagnosed, discussion should begin about the likely course of the disease (upper airway obstruction, respiratory failure, or both) and available treatment options. (3) Continued efforts must be made to resolve conflicts between healthcare professionals and surrogates concerning aggressive support of children with severe CNS dysfunction. Discussions should continue even after a decision to provide long-term ventilation is made. (4) Currently, requests by surrogates for LTV in patients with severe neurologic impairment are usually honored because of respect for family values. (5) Physicians and other healthcare professionals should develop an open and fair process for determining inappropriate care. (6) Once LTV is initiated, efforts to transfer the child to home or a long-term care facility should be made. Further life-saving support should be discouraged. (7) Irremediable patient suffering is reason to refuse a surrogate request for LTV. A patient's preservable existence might be so torturous, painful, or filled with suffering that continued medical intervention would be inhumane or abusive.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Cell surface expression of class II MHC proteins bound by a single peptide

On normal cells, the peptide-binding grooves of class II MHC proteins contain a wide spectrum of peptides. For some purposes, however, it would be helpful to have cells bearing class II proteins engaged by only one peptide species. In an attempt to make such cells we constructed a gene for a MHC class II beta-chain, IA beta b, covalently linked to a peptide, E alpha 52-68, which is known to bind to the peptide-binding groove of IAb. This gene, together with the gene for IA alpha b, was transfected into B lymphoma cells and fibroblasts. The IAb-E alpha complex was expressed on the surfaces of these cells where it could be recognized by a mAb and T cells specific for IAb plus E alpha 52-68. Most of the peptide on fibroblasts remained covalently attached to the IAb beta-chain, but the covalent linker and/or peptide were degraded to some extent on B lymphoma cells. Nearly all of the IAb expressed by transfected fibroblasts was occupied by the E alpha peptide. Of 16 IAb-reactive T cell hybridomas, only 3 could respond to the IAb-E alpha complex on fibroblasts, confirming the idea that recognition of MHC may often involve recognition of the peptides bound to the MHC as well.

Cell surface expression of class II MHC proteins bound by a single peptide

On normal cells, the peptide-binding grooves of class II MHC proteins contain a wide spectrum of peptides. For some purposes, however, it would be helpful to have cells bearing class II proteins engaged by only one peptide species. In an attempt to make such cells we constructed a gene for a MHC class II beta-chain, IA beta b, covalently linked to a peptide, E alpha 52-68, which is known to bind to the peptide-binding groove of IAb. This gene, together with the gene for IA alpha b, was transfected into B lymphoma cells and fibroblasts. The IAb-E alpha complex was expressed on the surfaces of these cells where it could be recognized by a mAb and T cells specific for IAb plus E alpha 52-68. Most of the peptide on fibroblasts remained covalently attached to the IAb beta-chain, but the covalent linker and/or peptide were degraded to some extent on B lymphoma cells. Nearly all of the IAb expressed by transfected fibroblasts was occupied by the E alpha peptide. Of 16 IAb-reactive T cell hybridomas, only 3 could respond to the IAb-E alpha complex on fibroblasts, confirming the idea that recognition of MHC may often involve recognition of the peptides bound to the MHC as well.

Co-stimulation via CD28 induces activation of a refractory subset of MRL-lpr/lpr T lymphocytes

Peripheral lymphoid tissues of lpr mice contain a large proportion of TCR alpha beta/CD3+CD4-CD8- T cells that lack surface CD2 and express the B cell isoform of CD45, B220. This subset of T cells does not proliferate or produce IL-2 in response to mitogenic signals or TCR-CD3 ligation. At the same time, these abnormal T cells display several characteristics of an activated phenotype. Collectively, these properties of lpr CD4-CD8- T cells have functional parallels with anergic T cells. A critical co-stimulatory molecule implicated in the prevention of or recovery from anergy is CD28, which binds the ligand BB1/B7 on certain accessory cells. lpr CD4-CD8- T cells express normal levels of CD28 which is capable of transducing a strong proliferative signal to these cells in co-stimulation with mitogens. However, proliferation of lpr CD4-CD8- T cells in response to CD28 co-stimulation does not reach the levels observed in normal T cells stimulated under similar conditions. Stimulation with anti-CD28 mAb in conjunction with phorbol myristate acetate and ionomycin promotes cell cycling in the CD2- subset of CD4-CD8- T cells, and results in a slight induction of CD2 levels during the course of the culture period. However, the majority of cells obtained at the end of the culture period remain TCR alpha beta+ CD4-CD8-, CD2low/- and B220high, similar to freshly isolated CD4-CD8- lpr T cells. In contrast, if IL-2 is included in the cultures, a strong shift toward a CD2+ phenotype is observed by a majority of the lpr T cells. Upon repeat stimulation, these lpr CD4-CD8- T cells can now proliferate in an IL-2-dependent manner when stimulated with only anti-CD3 mAb or mitogens, in the absence of exogenous IL-2 or anti-CD28 mAb. These data show that the hyporesponsiveness of lpr CD4-CD8- T cells does not result from a lack of CD28 expression, that it is not a fixed state, and that it can be reversed by the induction of cell cycling in the presence of IL-2. These observations extend the parallels between lpr CD4-CD8- T cells and anergic T cells.

Elevated production of interferon-gamma and interleukin 4 by mature T cells from autoimmune lpr mice correlates with Pgp-1 (CD44) expression

The cell surface glycoprotein, Pgp-1 (CD44), has been shown to be a marker of murine memory T lymphocytes. When activated, Pgp-1hi memory T cells produce strikingly higher amounts of interferon-gamma (IFN-gamma) than naive Pgp-1lo T cells, yet both subsets make similar levels of interleukin (IL)2. Whereas Pgp-1hi cells represent only 20%-25% of peripheral T cells from most strains, this marker is expressed by the vast majority (greater than 90%) of T cells from autoimmune MRL mice homozygous for the lymphoproliferation (lpr) gene. The massive lymphadenopathy that develops in lpr/lpr mice is composed of both non-mature (CD4-CD8-) T cells as well as a greatly expanded number (up to 300-fold) of mature (CD4+CD8-,CD4-CD8+) T cells. Paralleling the expression of high levels of Pgp-1, we find that compared to normal mouse T cells, the lpr mature T lymphocyte subsets are also very high producers on a per cell basis of IFN-gamma and, for the CD4+ subset, IL 4. Increased concentrations of IFN-gamma and IL 4 produced by large numbers of lpr Pgp-1hi mature T cells could contribute to the autoimmune syndrome in MRL lpr/lpr mice through the effects of these cytokines on augmenting MHC class II expression and production of certain classes of antibodies.

Intact antigen receptor-mediated generation of inositol phosphates and increased intracellular calcium in CD4 CD8 T lymphocytes from MRL lpr mice

The predominant T lymphocytes that accumulate in the peripheral lymphoid tissues of mice homozygous for the lpr gene bear the phenotype CD3+CD4-CD8-. By certain functional criteria these cells would appear to have impaired CD3-mediated signal transduction, in that they do not respond to alloantigen and produce little if any detectable IL-2 or other lymphokines. However, the signal pathway appears adequate for achieving other T cell functions, including induction of high affinity IL-2R, and thymic deletion. To clarify the basis of this seeming discrepancy, we examined transmembrane signal transduction in T cell subsets of lpr/lpr (lpr) and +/+ mice, as defined by increased [Ca2+]i and the generation of inositol phosphates (InsPs). Stimulation of lpr CD4-CD8- cells with anti-CD3 antibody produced prompt and sustained increases in the concentration of [C2+]i and in InsPs. Similar responses occurred in mature T cells from lpr and +/+ mice, except for the somewhat slower kinetics of their increased [Ca2+]i. In marked distinction to the anti-CD2-mediated response, Con A, even in high doses, could not stimulate any increase of [Ca2+]i in lpr CD4-CD8- cells, and only modest increases in InsPs. Mature T cells, whether of lpr or +/+ origin, yielded normal increased [Ca2+]i with Con A. The reason for the differences in signal transduction between anti-CD3 and Con A stimulation of lpr CD4-CD8- cells may relate to the absence of surface structures on these immature T cells that are required for activation by Con A but not by anti-CD3. The data demonstrate that the CD3 complex in lpr CD4-CD8- T cells can couple to phospholipase C to hydrolyze phosphoinositides. These activation properties of lpr CD4-CD8- T cells have interesting functional parallels to thymocytes at the time of thymic selection, as well as tolerance induction of mature T lymphocytes.

Intact antigen receptor-mediated generation of inositol phosphates and increased intracellular calcium in CD4 CD8 T lymphocytes from MRL lpr mice

The predominant T lymphocytes that accumulate in the peripheral lymphoid tissues of mice homozygous for the lpr gene bear the phenotype CD3+CD4-CD8-. By certain functional criteria these cells would appear to have impaired CD3-mediated signal transduction, in that they do not respond to alloantigen and produce little if any detectable IL-2 or other lymphokines. However, the signal pathway appears adequate for achieving other T cell functions, including induction of high affinity IL-2R, and thymic deletion. To clarify the basis of this seeming discrepancy, we examined transmembrane signal transduction in T cell subsets of lpr/lpr (lpr) and +/+ mice, as defined by increased [Ca2+]i and the generation of inositol phosphates (InsPs). Stimulation of lpr CD4-CD8- cells with anti-CD3 antibody produced prompt and sustained increases in the concentration of [C2+]i and in InsPs. Similar responses occurred in mature T cells from lpr and +/+ mice, except for the somewhat slower kinetics of their increased [Ca2+]i. In marked distinction to the anti-CD2-mediated response, Con A, even in high doses, could not stimulate any increase of [Ca2+]i in lpr CD4-CD8- cells, and only modest increases in InsPs. Mature T cells, whether of lpr or +/+ origin, yielded normal increased [Ca2+]i with Con A. The reason for the differences in signal transduction between anti-CD3 and Con A stimulation of lpr CD4-CD8- cells may relate to the absence of surface structures on these immature T cells that are required for activation by Con A but not by anti-CD3. The data demonstrate that the CD3 complex in lpr CD4-CD8- T cells can couple to phospholipase C to hydrolyze phosphoinositides. These activation properties of lpr CD4-CD8- T cells have interesting functional parallels to thymocytes at the time of thymic selection, as well as tolerance induction of mature T lymphocytes.

Mutations within the decoding site of Escherichia coli 16S rRNA: growth rate impairment, lethality and intragenic suppression

Several C----U transitions and small deletions were introduced into the conserved region centered on base C1400 in Escherichia coli 16S rRNA by in vitro mutagenesis. The mutations were placed within rrnB operons on multicopy plasmids under the transcriptional regulation of either the normal rrnB P1P2 promoters or the temperature-inducible PL promoter from bacteriophage lambda and introduced into E. coli hosts. When expressed from the P1P2 promoters, several of the mutant 16S rRNAs impaired cell growth while others, including one in which U replaced C at position 1400 within the ribosomal decoding site, had little or no effect on cell doubling time. However, C----U transitions at positions 1395 and 1407, as well as the deletion of C1400, appeared to render their hosts inviable. Cells in which these mutations were expressed from the lambdaPL promoter died within four generations after induction. Unexpectedly, the lethal phenotype was suppressed intragenically by replacement of G1505 with A, C or U. Suppression may alleviate a functional defect in 30S subunits containing the U1395, U1407 or deltaC1400 mutations.