Cell-mediated inhibition of proliferation and activation of alloreactive cytotoxic lymphocytes: maintenance of response potential of precursors and dissociation between proliferation and effector function of activated cytotoxic lymphocytes

Adherent layers of macrophages (M phi-c) generated in vitro from splenic precursors inhibit lymphoproliferative responses to mitogen and to alloantigen without inhibiting the production of interleukin-2 (IL-2). Analysis of spleen cells stimulated for 48 hr in the presence of M phi-c indicated that both blastogenesis (increased cell mass) and expression of IL-2 receptors (7D4 determinants) were reduced. Analysis of BrdU incorporation (frequency of S-phase cells) and total cellular DNA revealed that the M phi-c inhibited the progression from G1 to S phase of cell cycle. The M phi-c not only inhibited the proliferative response to alloantigen but also prevented the generation of alloreactive cytotoxic T cells. The M phi-c were shown not to inhibit CTL responses by eliminating the stimulators or by inactivating precursors or inducing suppressors. The M phi-c were affecting the induction of CTL activity since the M phi-c did not affect the expression of cytolytic activity by activated CTL. The M phi-c did inhibit the proliferation of the activated CTL, suggesting that although cytolytic activity can be expressed in G1 phase of cell cycle, the activation of cytolytic activity in CTL-P may require a G1 to S phase transition. The cells recovered from 5-day MLC incubated in the presence of M phi-c were fully capable of generating a subsequent CTL response. This is in contrast to cells recovered from unstimulated cultures (no M phi-c) which have lost the ability to generate CTL responses. The M phi-c therefore prevent the generation of CTL responses in a totally reversible fashion, so as to allow activation and proliferation of CTL-P which have been removed from the influence of the M phi-c. These observations are discussed in the context of the currently hypothesized role of tissue macrophages in microenvironmental regulation.     

Interstitial hybridization sites of the (TTAGGG)n telomeric sequence on the chromosomes of some North American hylid frogs.

Interstitial hybridization sites for the (TTAGGG)n telomeric repeat sequence were present in all seven species of hylid frogs examined and in a triploid hybrid between two of the species. Intra- and interspecific differences and similarities in hybridization sites agreed with what is known about the systematics of these species. Chromosome fusions, fissions, and inversions do not appear to have played a role in the evolution of the interstitial sites for the telomeric repeat in the species examined.     

Inhibition of IL 2 responsiveness of mitogen-stimulated lymphocytes by SMLR-generated plastic adherent suppressor cells

The proliferative response of spleen cells to concanavalin A (Con A) can be abrogated by plastic adherent suppressors generated in syngeneic mixed lymphocyte cultures of spleen cells. The addition of exogenously produced interleukins does not overcome the suppression, indicating that the suppressors are not simply competing for available growth factors. To examine the effect of the suppressors on IL 2 production and responsiveness, spleen cells were stimulated with Con A in the presence of the suppressors and assayed both for ability to produce IL 2 and for ability to bind IL 2. The culture supernates of suppressed spleen cells contained normal titers of biologically active IL 2, indicating that the suppressors do not inhibit IL 2 production or inactivate IL 2 after it is produced. Since IL 1 is required for IL 2 production, the failure of the suppressors to affect IL 2 production suggests that the suppressors have no effect on either production of, or responsiveness to, IL 1. The suppressed cells did, however, display a substantial reduction in ability to absorb IL 2 activity from a standard IL 2 preparation. The reduced frequency of IL 2 reactive cells did not appear to be due to a cytotoxic reaction insofar as no difference in viable cell recovery was noted between suppressed and nonsuppressed cultures at the time of assay. The suppressors therefore appear to inhibit the expression of IL 2 responsiveness without inhibiting the production of IL 2.     

The Evolution of Theoretically Useful Traits

The purely theoretical notion of fitness or optimality that is employed for instance in optimization theory has come under attack from those who think that only a more historically based notion of fitness could have a central role in evolutionary explanation. They argue that the key notion is proven usefulness rather than theoretical usefulness. This paper articulates a notion of theoretical usefulness and defends its role in functional evolutionary explanations.     

Development and application of a screening assay for glycoside phosphorylases

Glycoside phosphorylases (GPs) are interesting enzymes for the glycosylation of chemical molecules. They require only a glycosyl phosphate as sugar donor and an acceptor molecule with a free hydroxyl group. Their narrow substrate specificity, however, limits the application of GPs for general glycoside synthesis. Although an enzyme’s substrate specificity can be altered and broadened by protein engineering and directed evolution, this requires a suitable screening assay. Such a screening assay has not yet been described for GPs. Here we report a screening procedure for GPs based on the measurement of released inorganic phosphate in the direction of glycoside synthesis. It appeared necessary to inhibit endogenous phosphatase activity in crude Escherichia coli cell extracts with molybdate, and inorganic phosphate was measured with a modified phosphomolybdate method. The screening system is general and can be used to screen GP enzyme libraries for novel donor and acceptor specificities. It was successfully applied to screen a residue E649 saturation mutagenesis library of Cellulomonas uda cellobiose phosphorylase (CP) for novel acceptor specificity. An E649C enzyme variant was found with novel acceptor specificity toward alkyl β-glucosides and phenyl β-glucoside. This is the first report of a CP enzyme variant with modified acceptor specificity.