Protein synthesis and ribosome activation during the early stages of phytohemagglutinin lymphocyte stimulation

Dissecting the Landscape of Activated CMV-Stimulated CD4+ T Cells in Humans by Linking Single-Cell RNA-Seq With T-Cell Receptor Sequencing

CD4+ T cells are crucial in cytomegalovirus (CMV) infection, but their role in infection remains unclear. The heterogeneity and potential functions of CMVpp65-reactivated CD4+ T cell subsets isolated from human peripheral blood, as well as their potential interactions, were analyzed by single-cell RNA-seq and T cell receptor (TCR) sequencing. Tregs comprised the largest population of these reactivated cells, and analysis of Treg gene expression showed transcripts associated with both inflammatory and inhibitory functions. The detailed phenotypes of CMV-reactivated CD4+ cytotoxic T1 (CD4+ CTL1), CD4+ cytotoxic T2 (CD4+ CTL2), and recently activated CD4+ T (Tra) cells were analyzed in single cells. Assessment of the TCR repertoire of CMV-reactivated CD4+ T cells confirmed the clonal expansion of stimulated CD4+ CTL1 and CD4+ CTL2 cells, which share a large number of TCR repertoires. This study provides clues for resolving the functions of CD4+ T cell subsets and their interactions during CMV infection. The specific cell groups defined in this study can provide resources for understanding T cell responses to CMV infection.

The Use of B-Cell Polysome Profiling to Validate Novel RPL5 (uL18) and RPL26 (uL24) Variants in Diamond-Blackfan Anemia

Diamond-Blackfan anemia (DBA) is a rare bone marrow failure syndrome usually caused by heterozygous variants in ribosomal proteins (RP) and which leads to severe anemia. Genetic studies in DBA rely primarily on multigene panels that often result in variants of unknown significance. Our objective was to optimize polysome profiling to functionally validate new large subunit RP variants. We determined the optimal experimental conditions for B-cell polysome profiles then performed this analysis on 2 children with DBA and novel missense RPL5 (uL18) and RPL26 (uL24) variants of unknown significance. Both patients had reduced 60S and 80S fractions when compared with an unaffected parent consistent with a large ribosomal subunit defect. Polysome profiling using primary B-cells is an adjunctive tool that can assist in validation of large subunit RP variants of uncertain significance. Further studies are necessary to validate this method in patients with known DBA mutations, small RP subunit variants, and silent carriers.

Protein Translation Activity: A New Measure of Host Immune Cell Activation

We describe the in vivo ribopuromycylation (RPM) method, which uses a puromycin-specific Ab to fluorescently label ribosome-bound puromycylated nascent chains, enabling measurement of translational activity via immunohistochemistry or flow cytometry. Tissue staining provides a unique view of virus-induced activation of adaptive, innate, and stromal immune cells. RPM flow precisely quantitates virus-induced activation of lymphocytes and innate immune cells, and it provides a unique measure of immune cell deactivation and quiescence. Using RPM we find that high endothelial cells in draining lymph nodes rapidly increase translation in the first day of vaccinia virus infection. We also find a population of constitutively activated splenic T cells in naive mice and further that most bone marrow T cells activate 3 d after vaccinia virus infection. Bone marrow T cell activation is nonspecific, IL-12-dependent, and induces innate memory T cell phenotypic markers. Thus, RPM measures translational activity to uniquely identify cell populations that participate in the immune response to pathogens, other foreign substances, and autoantigens.

Changes in ribosomal binding activity of eIF3 correlate with increased translation rates during activation of T lymphocytes

The rate of protein synthesis in quiescent peripheral blood T lymphocytes increases dramatically following mitogenic activation. The stimulation of translation is due to an increase in the rate of initiation caused by the regulation of initiation factor activities. Here, we focus on eIF3, a large multiprotein complex that plays a central role in the formation of the 40 S initiation complex. Using sucrose density gradient centrifugation to analyze ribosome complexes, we find that most eIF3 is not bound to 40 S ribosomal subunits in unactivated T lymphocytes but becomes ribosome-bound following activation. Immunoblot analyses of sucrose gradient fractions for individual eIF3 subunits show that the small eIF3j subunit is unassociated with the eIF3 complex in quiescent T lymphocytes, but upon activation joins the other eIF3 subunits and binds 40 S ribosomal subunits. Because eIF3j has been shown to be required for eIF3 binding to 40 S ribosomes in vitro, the results suggest that mitogenic stimulation of T lymphocytes leads to an activation of eIF3j, thereby enabling eIF3 to bind to the larger ribosome-free eIF3 subunit complex, and then to the 40 S ribosomes. The association of eIF3j with the other eIF3 subunits appears to be inhibited by rapamycin, suggesting a mechanism that lies downstream from the mammalian target of rapamycin kinase. This association requires ionomycin together with a phorbol ester, which also suggests that calcium signaling is involved. We conclude that the complex formation of eIF3 and its association with the ribosomes might contribute to increased translation rates during T lymphocyte activation.

Translational control: a general mechanism for gene regulation during T cell activation

Distributional changes of individual mRNAs between free ribonucleoprotein particles (mRNP) and ribosome-bound transcripts are used to assess translational control. Simultaneous analysis of many mRNA species is required to estimate the overall contribution of translation to the regulation of gene expression. To this purpose, total cytoplasmic RNA was fractionated in sucrose step gradients and poly(A)+ RNA was prepared from mRNP and ribosome-bound fractions. Since direct, simultaneous analysis of a profusion of mRNAs is not feasible, distribution of their in vitro translation products was examined after separation in 2-dimensional gels, followed by computer-based analysis of autoradiographs. When this analysis was applied to antigenically stimulated T cells, 36% of in vitro translation products showed a greater than 10-fold increase in intensity, suggesting transcriptional activation of the corresponding mRNAs. In comparison, 7.9% of individual mRNAs (54 of 685 species) were translationally activated. They were redistributed from free mRNP to ribosome-associated fractions; 4.7% (32 species) were translationally repressed, as indicated by the opposite pattern. The differential recruitment of 12.6% of mRNA species demonstrates specificity and the general significance of translational control during T cell activation, which implies that translation may play a similar role in regulating gene expression in a variety of physiological processes.

T-cell activation leads to rapid stimulation of translation initiation factor eIF2B and inactivation of glycogen synthase kinase-3

Mitogenic stimulation of T-lymphocytes causes a rapid activation or protein synthesis, which reflects in part increased expression of many translation components. Their levels, however, rise more slowly than the rate of protein synthesis, indicating an enhancement of the efficiency of their utilization. Initiation factor eIF2B catalyzes a key regulatory step in the initiation of translation, and we have therefore studied its activity following T-cell activation. eIF2B activity rises quickly, increasing as early as 5 min after cell stimulation. This initial phase is followed by an additional slow but substantial increase in eIF2B activity. The level of eIF2B subunits did not change over the initial rapid phase but did increase at later time points. Northern analysis revealed that levels of eIF2B mRNA only rose during the later phase. The rapid activation of EIF2B following mitogenic stimulation of T-cells is therefore mediated by factors other than its own concentration. The largest (epsilon) subunit of eIF2B is a substrate for glycogen synthase kinase-3 (GSK-3), the activity of which rapidly decreases following T-cell activation. Since phosphorylation of eIF2B by GSK-3 appears to inhibit nucleotide exchange in vitro, this provides a potential mechanism by which eIF2B may be activated.

Regulation of eukaryotic translation initiation factor expression during T-cell activation

Primary T-cells are metabolically quiescent, with little DNA, RNA or protein synthesis. Upon mitogenic stimulation the rate of protein synthesis increases 10-fold. We have studied the role of eIF-2 and eIF-4 alpha (eIF-4E) expression in the mechanism of translational activation. During this period, the levels of eIF-2 alpha and eIF-4 alpha mRNA increase some 50-fold. Similar to the increase in ribosomes and mRNA, the number of eIF-2 alpha, eIF-2 beta, and eIF-4 alpha molecules per cell also increase 2-3-fold. This suggests that in addition to an increase in the pool size of translational components, an additional mechanism exists which results in an increased efficiency of factor utilization. We have looked at initiation factor phosphorylation. We find that eIF-2 alpha does not undergo significant changes in its phosphorylation state nor is there a change in the efficiency of eIF-2 utilization. However, there is a rapid increase in the phosphorylation state of eIF-4 alpha which correlates with the rapid increase in translational activity. It thus appears there are 2 distinct components responsible for the translational activation of quiescent T-cells during mitogenic stimulation. The first is the phosphorylation of eIF-4 alpha, with a concomitant increase in the efficiency of eIF-4 alpha utilization. The second is an increase in the pool sizes of eIF-2 and eIF-4 alpha.

Age-related influence of piracetam on mitotic index and number of silver-stained nucleolus organizer regions

Mitotic indices and the number of silver-positive nucleolus organizer regions (AgNORs) were scored in phytohemagglutinin-stimulated cultures of peripheral lymphocytes from two age groups of females (mean = 23.1 and 84.0 yr, respectively) under the influence of Piracetam (2-oxo-pyrrolidine-1-acetamid; Nootropil, Reg. No. 17051) and in simultaneously set up control cultures without Piracetam addition. Piracetam concentrations of 10, 14 and 16 mg/ml culture medium produced a highly significant, decreasing effect on both parameters tested, without an age-related difference. Lower Piracetam concentrations (2 and 4 mg/ml culture medium) showed a depressant effect on some of the cultures only; but, on average, there was a rather equal, significant, dose-dependent, linear decrease of the mitotic indices of both age groups, whereas the suppressive effect on the number of AgNORs was significant in cultures from the young females only.

Modulation of human lymphocyte proliferation by amino acids

The amino acids required for phytohaemagglutinin (PHA) induced lymphocyte proliferation were determined by the 3H-thymidine incorporation in amino acid-deficient media. Results indicate that the PHA-stimulated lymphocytes require alanine and serine in addition to 13 other amino acids present in Eagle's minimal essential medium (arginine, cysteine, glutamine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, tyrosine, and valine). The omission of any one of the 13 amino acids would stop almost completely the proliferation of PHA-stimulated lymphocytes. The omission of serine from RPMI 1640 medium caused a mean reduction of 64% of cell proliferation, while the addition of alanine to PRMI 1640 culture medium caused a mean increment of 52%. The lymphocyte proliferation appears to be modulated by amino acids in the culture medium, and for optimal growth of lymphocytes, all these 15 amino acids are essential.

Interchromatin granules during phytohemagglutinin stimulation of human lymphocytes. A cytochemical study

We studied the formation of interchromatin granules (IGs) in phytohemagglutinin (PHA)-stimulated lymphocytes. The bismuth staining method was used for the visualization of IGs, and we also applied high-resolution autoradiography after incubating cells in the presence of 3H-leucine during different stages of lymphocyte activation. The disaggregation of chromatin and the enlargement of interchromatinic areas in stimulated lymphocytes were found to be accompanied by an increase in the number of IGs, and it was shown that IGs were formed during all of the investigated stages of lymphocyte stimulation.

Inhibition of T-lymphocyte activation by amiloride

The T-lymphocyte activation process involves a series of coordinately coupled biochemical events occurring in response to antigen or mitogen. These events have not been completely characterized. The present studies investigate the mechanism of protein synthesis during the initial phase of T-cell activation. Among the early biochemical changes, induction of protein synthesis was observed as early as 10 minutes after mitogen stimulation of T-lymphocytes. This early protein synthesis was inhibited by cycloheximide but was insensitive to actinomycin-D, indicating the presence of preformed mRNA in resting lymphocytes. Since early protein synthesis parallels the increase in protein kinase C activity in activated T-lymphocytes, these two biochemical events may be related. In the present report, amiloride, an inhibitor of Na+/H+ antiport and protein kinase C, significantly inhibited [3H]leucine and [3H]thymidine incorporation in a dose-dependent manner into phytohemagglutinin (PHA)-stimulated T-lymphocytes. Furthermore, when T-lymphocytes were stimulated by phorbol myristate acetate, a known activator of protein kinase C, a similar inhibition of protein and DNA synthesis by amiloride was observed. The partially purified cytosol fraction isolated from PHA-activated T-lymphocytes showed a 75% decrease in protein kinase C-mediated [32P] incorporation from ATP in the presence of 100 microM amiloride. These results suggest that the T-cell activation process following exposure to mitogens involves early protein synthesis, which may be mediated by protein kinase C.

Effect of polymyxin-B on T-lymphocyte protein synthesis

The role of protein kinase-C (PK-C) protein phosphorylation on the mitogen triggered responses of T-lymphocytes was examined by observing the effect of polymyxin-B (an inhibitor of PK-C) on mitogen induced protein and DNA synthesis. Polymyxin-B inhibited 3H-thymidine incorporation by PHA activated T-lymphocytes over a range of PHA concentrations. 3H-leucine incorporation by PHA activated T-lymphocytes was inhibited by polymyxin-B in a dose dependent manner. A partially purified PK-C fraction from polymyxin-B treated PHA activated T-lymphocytes demonstrated less than 25% of the phosphorylating activity of untreated lymphocytes. These results suggest that protein synthesis during the T-lymphocyte activation process is dependent on PK-C activity.

Expression of intracellular biochemical defects of lymphocytes in aging: proposal of a general aging mechanism which is not cell-specific

There is a decline in immune capacity with age which is expressed on the organismic level by association with numerous immune-related diseases, on the cellular level by impaired mitogenesis, on the biochemical level by impaired metabolic pathways, and on the molecular level by decreased protein synthesis and degradation. Defects in various cofactors such as calcium and several nucleotides also occur and may be related to the impaired enzyme function during mitogenesis in the aged. The central cause for decreased mitogenesis in the aged could be a decrease in protein synthesis which appears to cause impaired enzyme induction. This impaired enzyme induction accounts in part for the decreased glycolytic flux and DNA synthesis in these cells. Decreased protein synthesis also has been associated with a decreased synthesis of lymphokines which help these cells to proliferate. Numerous other intracellular age-related defects of lymphocytes also occur which may collectively play important interdependent roles in the impaired lymphocyte function of the aged. A potential general underlying mechanism of cellular senescence is proposed based on a genetic "slowing-cycle" effect of transcription, translation, and enzyme induction with immunosenescence presented as an example of an expression of these basic defects.

A simple microplate assay for the determination of cellular protein

In assays of leukocyte function it is desirable to correlate a specific measurement, e.g., the release of enzymes, the production of metabolites or the secretion of cytokines with the amount of cellular material. Many authors use either the protein or the DNA content of the cells as a reference. Specific measurements of cellular activity are often expressed as a ratio of activity/protein or activity/DNA. This communication describes a dye-binding assay which allows the determination of total protein of the same cell sample which was first used for the quantitation of a specific cellular function. The microplate assay described is an adaptation of the Coomassie blue dye-binding assay without the need of a separate step to lyse the cells. The test is useful for both adherent and non-adherent cells and has a detection limit of less than 500 ng BSA or 5000 leukocytes.

Chromatin structure and nucleic acid synthesis in human lymphocyte activation by phytohemagglutinin

Alterations in chromatin structure were explored as a function of time following phytohemagglutinin (PHA) activation of human lymphocytes. Nuclear refringency (related to chromatin status), ultrastructural changes, and modifications of nucleic acid synthesis were compared. Chromatin dispersion was observed within 20 min of stimulation, with a concomitant increase in [3H]uridine incorporation; both were inhibited by actinomycin D. Recondensation of chromatin had occurred at 60 min when RNA synthesis was still increasing. The early chromatin dispersion thus was a reversible step and one of the most striking cellular events during the first phase of PHA activation. Late chromatin dispersion of blastic cells during the second phase of PHA response (after 24 h) was related to DNA synthesis in mitotic cells. This step was also inhibited by actinomycin D. These results suggest a relationship between the morphological changes in chromatin structure and the induction of RNA and DNA syntheses. The regulation of transcriptional and replicational states of the nucleus is discussed in terms of modifications of chromatin structure, considering PHA-induced calcium and cyclic nucleotide metabolism and interleukin II receptor expression.

Distribution of lymphocyte messenger RNA during stimulation by phytohaemagglutinin

Additional of phytohaemagglutinin to cultured lymphocytes results in a progressive increase in the rate of protein synthesis of up to 7-10 times the initial rate after 20 h. Between 2 h and 12 h after the addition of mitogen, the increase in the rate of protein synthesis can be accounted for by the transfer of mRNA from messenger ribonucleoprotein complexes to polysomes. Between 12 h and 20 h the increase also reflects the accumulation of mRNA and ribosomes. However, the proportion of mRNA associated with ribosomes in unstimulated lymphocytes is unexpectedly high and paradoxically decreases during the first 2 h after mitogen addition, although the rate of protein synthesis increases. A mechanism involving mRNA selection is suggested.

Free fibosomes and growth stimulation in human peripheral lymphocytes: activation of free ribisomes as an essential event in growth induction

During the initial ten hours of growth in lymphocytes stimulated by phytohemagglutinin, the cells are converted from a state in which over 70% of all ribosomes are inactive free ribosomes, to one in which over 80% of ribosomes are in polysomes or in native ribosomal subunits. In this initial period, there was a neglible increase in total ribosomal RNA due to increased RNA synthesis, and abolition of ribosomal RNA synthesis with low concentrations of actinomycin D did not interfere with polysome formation. Therefore, the conversion is accomplished by the activation of existing free ribosomes rather than by accumulation of newly synthesized particles. The large free ribosome pool of resting lymphocytes is thus an essential source of components for accelerated protein synthesis early in lymphocyte activation, before increased synthesis can provide a sufficient number of new ribosomes. Free ribosomes accumulate once more after 24 to 48 hours of growth, when RNA and DNA synthetic activity are maximal. This reaccumulation of inactive ribosomes at the peak of growth activity may represent preparation for a return to the resting state where cells are again susceptible to stimulation. Activation of free ribosomes to form polysomes appears to involve modification of at least two steps: (a) dissociation of free ribosomes with stabilization as native subunits, and (b) adjustment of a rate-limiting step at initiation.

Protein synthesis in resting and stimulated human lymphocytes

The ribosomal profiles in lysates from resting and phytohemagglutinin stimulated human lymphocytes have been analyzed by sucrose gradient centrifugation. The percentage of polyribosomes increased during lymphocyte transformation reaching a maximal value of 60 to 70% of the total ribosomes after 72 hours of mitogen addition. This time period coincides with maximal in vivo protein synthesis. On the other hand, in nonstimulated lymphocytes, about 25% of the ribosomal particles appeared as aggregates, independently of the incubation period. Experiments performed with homologous cell free systems containing ribosomes and supernatant fluids prepared from unstimulated or activated lymphocytes demonstrate that the mixtures containing both components from stimulated lymphocytes are several fold more active in polypeptide synthesis than the systems which contain ribosomal particles and cell sap from resting cells. Assays carried out with mixtures combining the components from both sources indicate that the increased activity depends on ribosomes as well as on the supernatant fractions.