Prolonged activation in CD4+ T cells results in extensive mitochondrial remodeling despite the metabolic dominance of aerobic glycolysis

Activation of CD4+ T cells to proliferate drives cells toward aerobic glycolysis for energy production while using mitochondria primarily for macromolecular synthesis. In addition, the mitochondria of activated T cells increase production of reactive oxygen species, providing an important second messenger for intracellular signaling pathways. To better understand the critical changes in mitochondria that accompany prolonged T cell activation, we carried out an extensive analysis of mitochondrial remodeling using a combination of conventional strategies and a novel high-resolution imaging method. We show that for four days following activation, mouse CD4+ T cells sustained their commitment to glycolysis facilitated by increased glucose uptake through increased expression of GLUT transporters. Despite their limited contribution to energy production, mitochondria were active and showed increased reactive oxygen species production. Despite elevated ROS production, it was successfully counterbalanced by antioxidant enzymes, hence the mitochondria did not display signs of oxidative stress as evidenced by the healthy cristae structure and overall morphology as well as the maintained mitochondrial reserve. Moreover, prolonged activation of CD4+ T cells led to increases in mitochondrial content and volume, in the number of mitochondria per cell and in mitochondrial biogenesis. Thus, during prolonged activation, CD4+ T cells continue to obtain energy predominantly from glycolysis but also undergo extensive mitochondrial remodeling, resulting in increased mitochondrial activity.

Regulatory T cells perform antigen specific suppression by depleting cognate peptide-MHC class II via trogocytosis

Tregs are professional suppressors of the immune response, yet their mechanism of action in vivo remains unclear. Using electron microscopy and dynamic intravital two-photon microscopy, we revealed that unlike effector T cells, Tregs displayed a distinct morphology with membrane projections that resemble filopodia at the DC binding site increasing the surface area of the contact dramatically. Despite the intense contact with DC, Tregs only abolished the interactions of effector T cells with DC when the effectors shared the same antigen specificity with Tregs. However, this cessation of the effector T cell interactions was not due to a physical hindrance created by the superior binding ability of Treg since the antigen specific inhibition of T cell-DC interaction was still observed after Tregs are removed from the environment. Instead, the antigen-specific inhibition was secondary to the removal of cognate peptide-MHC Class II (pMHCII) from the surface of DC as evidenced by the strictly antigen-specific depletion of the pMHCII from double antigen pulsed DC. pMHCII capture was only performed by antigen specific Treg, not by polyclonal Treg and it was not secondary to the interaction of CTLA-4 with B7 molecules suggesting the functional importance of TCR in Treg suppression. Prominent members of the B7 family, that are not de novo expressed in Tregs, such as CD86, ICOSL and PD-L2 were acquired alongside the TCR, suggestive of a process that includes a membrane transfer event called trogocytosis. Altogether, we propose antigen specific depletion of peptide-MHCII complexes via trogocytosis as a new mechanism for Treg-mediated suppression.

Tregs orchestrate antigen specific suppression via stripping cognate peptide-MHCII from the DC surface

Tregs are professional suppressors of the immune response, yet their mechanism of action in vivo remains unclear. We compared the morphology and stoichiometry of the interactions of activated 5CC7 T cells and 5CC7 iTregs with MCC88–103 pulsed splenic DCs by electron and confocal microscopy. Image analyses revealed that Tregs interacting with DC displayed a distinct morphology with finger like membrane projections at the DC binding site and uropods at the rear end within three hours of in vitro culture. In contrast, activated T cells maintained their round morphology. Tregs occupied a greater extent of the DC surface than activated T cells consistent with a higher binding avidity. Dynamic intravital two-photon microscopy of adoptively transferred OTII iTregs and activated OTII cells demonstrated that Treg displayed a greater volume and duration of contact with OVA323–339 pulsed DC compared to that of activated OTII cells. Subsequent to their high avidity interactions with DC, 5CC7 iTregs captured MCC88–103-I-Ek complexes from DC surface reducing the amount of MCC presented on DC. Reduced antigen presentation was not due to global suppression of the DC’s ability to present antigen as 5CC7 and 3A9 iTregs only captured their cognate peptide-MHCII from MCC/HEL double pulsed DC, reducing the DC presentation in an antigen specific manner. When double pulsed DCs were cultured with iTregs specific for one peptide, separated from the iTregs, the DC failed to prime T cells specific for the antigen seen by the iTreg, but primed T cells specific for the second antigen. Altogether, we propose antigen specific depletion of peptide-MHCII complexes as a new mechanism for Treg-mediated suppression.

Structural alterations in a type IV pilus subunit protein result in concurrent defects in multicellular behaviour and adherence to host tissue

The ability of bacteria to establish complex communities on surfaces is believed to require both bacterial-substratum and bacterial-bacterial interactions, and type IV pili appear to play a critical but incompletely defined role in both these processes. Using the human pathogen Neisseria gonorrhoeae, spontaneous mutants defective in bacterial self-aggregative behaviour but quantitatively unaltered in pilus fibre expression were isolated by a unique selective scheme. The mutants, carrying single amino acid substitutions within the conserved amino-terminal domain of the pilus fibre subunit, were reduced in the ability to adhere to a human epithelial cell line. Co-expression of the altered alleles in the context of a wild-type pilE gene confirmed that they were dominant negative with respect to aggregation and human cell adherence. Strains expressing two copies of the altered alleles produced twice as much purifiable pili but retained the aggregative and adherence defects. Finally, the defects in aggregative behaviour and adherence of each of the mutants were suppressed by a loss-of-function mutation in the twitching motility gene pilT. The correlations between self-aggregation and the net capacity of the microbial population to adhere efficiently demonstrates the potential significance of bacterial cell-cell interactions to colonization.

Components and dynamics of fiber formation define a ubiquitous biogenesis pathway for bacterial pili

Type IV pili (Tfp) are a unique class of multifunctional surface organelles in Gram-negative bacteria, which play important roles in prokaryotic cell biology. Although components of the Tfp biogenesis machinery have been characterized, it is not clear how they function or interact. Using Neisseria gonorrhoeae as a model system, we report here that organelle biogenesis can be resolved into two discrete steps: fiber formation and translocation of the fiber to the cell surface. This conclusion is based on the capturing of an intermediate state in which the organelle is retained within the cell owing to the simultaneous absence of the secretin family member and biogenesis component PilQ and the twitching motility/pilus retraction protein PilT. This finding is the first demonstration of a specific translocation defect associated with loss of secretin function, and additionally confirms the role of PilT as a conditional antagonist of stable pilus fiber formation. These findings have important implications for Tfp structure and function and are pertinent to other membrane translocation systems that utilize a highly related set of components.

Porin polypeptide contributes to surface charge of gonococci

Each strain of Neisseria gonorrhoeae elaborates a single porin polypeptide, with the porins expressed by different strains comprising two general classes, Por1A and Por1B. In the outer membrane, each porin molecule folds into 16 membrane-spanning beta-strands joined by top- and bottom-loop domains. Por1A and Por1B have similar membrane-spanning regions, but the eight surface-exposed top loops (I to VIII) differ in length and sequence. To determine whether porins, and especially their top loop domains, contribute to bacterial cell surface charge, strain MS11 gonococci that were identical except for expressing a recombinant Por1A, Por1B, or mosaic Por1A-1B polypeptide were compared by whole-cell electrophoresis. These porin variants displayed different electrophoretic mobilities that correlated with the net numbers of charged amino acids within surface-exposed loops of their respective porin polypeptides. The susceptibilities of porin variants to polyanionic sulfated polymers correlated roughly with gonococcal surface charge; those porin variants with diminished surface negativity showed increased sensitivity to the polyanionic sulfated compounds. These observations indicate that porin polypeptides in situ contribute to the surface charge of gonococci, and they suggest that the bacterium's interactions with large sulfated compounds are thereby affected.

In vitro effects of anti-HIV immunotoxins directed against multiple epitopes on HIV type 1 envelope glycoprotein 160

We have used a panel of anti-gp160 MAbs to construct anti-HIV immunotoxins by coupling antibodies to ricin A chain (RAC). The ability of the immunotoxins to kill HIV-1-infected cells and halt the spread of infection was tested in tissue culture on persistently and acutely infected cell lines and primary lymphocyte cultures stimulated with phytohemagglutinin (PHA blasts). Laboratory strains and clinical isolates of HIV both were tested. The constitution and antigen-binding capacity of the immunotoxins were confirmed by ELISA and indirect immunofluorescence. Immunotoxins that bind epitopes exposed on the cell surface effectively killed persistently infected cells, although killing was not directly proportional to binding of immunotoxin to cell. The activity of anti-gp41, but not anti-gp120, immunotoxins was markedly enhanced in the presence of soluble CD4 or peptides corresponding to the CDR3 region of CD4. CD4-mediated enhancement of anti-gp41 immunotoxin activity was observed for laboratory strains neutralized by sCD4 and for clinical isolates that were resistant to neutralization by sCD4. Immunotoxin action was potentiated by brefeldin A, bafilomycin A1, cortisone, and an amphipathic fusion peptide, but not by cytochalasin D, nocodazol, monodansyl cadaverine, or trans-retinoic acid. Anti-HIV immunotoxins are useful tool with which to study the functional expression of gp120/gp41 antigens on the surface of HIV-infected cells, as well as potential AIDS therapeutics. Because these studies relate to the accessibility of viral antigens to antibody-mediated attack, these studies also have relevance for vaccine development.