Lymphocytes and Macrophages Are Infected by Theileria equi,but T Cells and B Cells Are Not Required to Establish Infection In Vivo

Theileria equi has a biphasic life cycle in horses, with a period of intraleukocyte development followed by patent erythrocytic parasitemia that causes acute and sometimes fatal hemolytic disease. Unlike Theileria spp. that infect cattle (Theileria parva and Theileria annulata), the intraleukocyte stage (schizont) of Theileria equi does not cause uncontrolled host cell proliferation or other significant pathology. Nevertheless, schizont-infected leukocytes are of interest because of their potential to alter host cell function and because immune responses directed against this stage could halt infection and prevent disease. Based on cellular morphology, Theileria equi has been reported to infect lymphocytes in vivo and in vitro, but the specific phenotype of schizont-infected cells has yet to be defined. To resolve this knowledge gap in Theileria equi pathogenesis, peripheral blood mononuclear cells were infected in vitro and the phenotype of infected cells determined using flow cytometry and immunofluorescence microscopy. These experiments demonstrated that the host cell range of Theileria equi was broader than initially reported and included B lymphocytes, T lymphocytes and monocyte/macrophages. To determine if B and T lymphocytes were required to establish infection in vivo, horses affected with severe combined immunodeficiency (SCID), which lack functional B and T lymphocytes, were inoculated with Theileria equi sporozoites. SCID horses developed patent erythrocytic parasitemia, indicating that B and T lymphocytes are not necessary to complete the Theileria equi life cycle in vivo. These findings suggest that the factors mediating Theileria equi leukocyte invasion and intracytoplasmic differentiation are common to several leukocyte subsets and are less restricted than for Theileria annulata and Theileria parva. These data will greatly facilitate future investigation into the relationships between Theileria equi leukocyte tropism and pathogenesis, breed susceptibility, and strain virulence.     

The Roles of Competition and Mutation in Shaping Antigenic and Genetic Diversity in Influenza

Influenza A (H3N2) offers a well-studied, yet not fully understood, disease in terms of the interactions between pathogen population dynamics, epidemiology and genetics. A major open question is why the virus population is globally dominated by a single and very recently diverged (2–8 years) lineage. Classically, this has been modeled by limiting the generation of new successful antigenic variants, such that only a small subset of progeny acquire the necessary mutations to evade host immunity. An alternative approach was recently suggested by Recker et al. in which a limited number of antigenic variants are continuously generated, but most of these are suppressed by pre-existing host population immunity. Here we develop a framework spanning the regimes described above to explore the impact of rates of mutation and levels of competition on phylodynamic patterns. We find that the evolutionary dynamics of the subtype H3N2 influenza is most easily generated within this framework when it is mutation limited as well as being under strong immune selection at a number of epitope regions of limited diversity.     

An essential role for the ATG8 ortholog LC3C in antibacterial autophagy

Autophagy defends the mammalian cytosol against bacterial invasion. Efficient bacterial engulfment by autophagy requires cargo receptors that bind (a) homolog(s) of the ubiquitin-like protein Atg8 on the phagophore membrane. The existence of multiple ATG8 orthologs in higher eukaryotes suggests that they may perform distinct functions. However, no specific role has been assigned to any mammalian ATG8 ortholog. We recently discovered that the autophagy receptor CALCOCO2/NDP52, which detects cytosol-invading Salmonella enterica serovar Typhimurium (S. Typhimurium), preferentially binds LC3C. The CALCOCO2/NDP52-LC3C interaction is essential for cell-autonomous immunity against cytosol-exposed S. Typhimurium, because cells lacking either protein fail to target bacteria into the autophagy pathway. The selectivity of CALCOCO2/NDP52 for LC3C is determined by a novel LC3C interacting region (CLIR), in which the lack of the key aromatic residue of canonical LIRs is compensated by LC3C-specific interactions. Our findings provide a new layer of regulation to selective autophagy, suggesting that specific interactions between autophagy receptors and the ATG8 orthologs are of biological importance.     

Seasonal and Geographic Variation of Southern Blue Whale Subspecies in the Indian Ocean

Understanding the seasonal movements and distribution patterns of migratory species over ocean basin scales is vital for appropriate conservation and management measures. However, assessing populations over remote regions is challenging, particularly if they are rare. Blue whales (Balaenoptera musculus spp) are an endangered species found in the Southern and Indian Oceans. Here two recognized subspecies of blue whales and, based on passive acoustic monitoring, four “acoustic populations” occur. Three of these are pygmy blue whale (B.m. brevicauda) populations while the fourth is the Antarctic blue whale (B.m. intermedia). Past whaling catches have dramatically reduced their numbers but recent acoustic recordings show that these oceans are still important habitat for blue whales. Presently little is known about the seasonal movements and degree of overlap of these four populations, particularly in the central Indian Ocean. We examined the geographic and seasonal occurrence of different blue whale acoustic populations using one year of passive acoustic recording from three sites located at different latitudes in the Indian Ocean. The vocalizations of the different blue whale subspecies and acoustic populations were recorded seasonally in different regions. For some call types and locations, there was spatial and temporal overlap, particularly between Antarctic and different pygmy blue whale acoustic populations. Except on the southernmost hydrophone, all three pygmy blue whale acoustic populations were found at different sites or during different seasons, which further suggests that these populations are generally geographically distinct. This unusual blue whale diversity in sub-Antarctic and sub-tropical waters indicates the importance of the area for blue whales in these former whaling grounds.     

Gender-Mediated Differences in Vertical Transmission of a Nucleopolyhedrovirus

With the development of sensitive molecular techniques for detection of low levels of asymptomatic pathogens, it becoming clear that vertical transmission is a common feature of some insect pathogenic viruses, and likely to be essential to virus survival when opportunities for horizontal transmission are unfavorable. Vertical transmission of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) is common in natural populations of S. exigua. To assess whether gender affected transgenerational virus transmission, four mating group treatments were performed using healthy and sublethally infected insects: i) healthy males (H♂)×healthy females (H♀); ii) infected males (I♂)×healthy females (H♀); iii) healthy males (H♂)×infected females (I♀) and iv) infected males (I♂)×infected females (I♀). Experimental adults and their offspring were analyzed by qPCR to determine the prevalence of infection. Both males and females were able to transmit the infection to the next generation, although female-mediated transmission resulted in a higher prevalence of infected offspring. Male-mediated venereal transmission was half as efficient as maternally-mediated transmission. Egg surface decontamination studies indicated that the main route of transmission is likely transovarial rather than transovum. Both male and female offspring were infected by their parents in similar proportions. Incorporating vertically-transmitted genotypes into virus-based insecticides could provide moderate levels of transgenerational pest control, thereby extending the periods between bioinsecticide applications.     

Skin in the game: Epidermal molt as a driver of long-distance migration in whales

Long-distance migration in whales has historically been described as an annual, round-trip movement between high-latitude, summer feeding grounds, and low-latitude, winter breeding areas, but there is no consensus about why whales travel to the tropics to breed. Between January 2009 and February 2016, we satellite-tagged 62 antarctic killer whales (Orcinus orca) of four different ecotypes, of which at least three made short-term (6–8 weeks), long-distance (maximum 11,000 km, round trip), essentially nonstop, migrations to warm waters (SST 20°C–24°C), and back. We previously suggested that antarctic killer whales could conserve body heat in subfreezing (to −1.9°C) waters by reducing blood flow to their skin, but that this might preclude normal (i.e., continuous) epidermal molt, and necessitate periodic trips to warm waters for routine skin maintenance (“skin molt migration,” SMM). In contrast to the century-old “feeding/breeding” migration paradigm, but consistent with a “feeding/molting” hypothesis, the current study provides additional evidence that deferred skin molt could be the main driver of long-distance migration for antarctic killer whales. Furthermore, we argue that for all whales that forage in polar latitudes and migrate to tropical waters, SMM might also allow them to exploit rich prey resources in a physiologically challenging environment and maintain healthy skin.     

Crystal structures of Escherichia coli exonuclease I in complex with single-stranded DNA provide insights into the mechanism of processive digestion

Escherichia coli Exonuclease I (ExoI) digests single-stranded DNA (ssDNA) in the 3′-5′ direction in a highly processive manner. The crystal structure of ExoI, determined previously in the absence of DNA, revealed a C-shaped molecule with three domains that form a central positively charged groove. The active site is at the bottom of the groove, while an extended loop, proposed to encircle the DNA, crosses over the groove. Here, we present crystal structures of ExoI in complex with four different ssDNA substrates. The structures all have the ssDNA bound in essentially the predicted manner, with the 3′-end in the active site and the downstream end under the crossover loop. The central nucleotides of the DNA form a prominent bulge that contacts the SH3-like domain, while the nucleotides at the downstream end of the DNA form extensive interactions with an ‘anchor’ site. Seven of the complexes are similar to one another, but one has the ssDNA bound in a distinct conformation. The highest-resolution structure, determined at 1.95 Å, reveals an Mg²⁺ ion bound to the scissile phosphate in a position corresponding to Mg^B in related two-metal nucleases. The structures provide new insights into the mechanism of processive digestion that will be discussed.     

A unifying conceptual model for the environmental responses of isoprene emissions from plants

Background and Aims

Isoprene is the most important volatile organic compound emitted by land plants in terms of abundance and environmental effects. Controls on isoprene emission rates include light, temperature, water supply and CO₂ concentration. A need to quantify these controls has long been recognized. There are already models that give realistic results, but they are complex, highly empirical and require separate responses to different drivers. This study sets out to find a simpler, unifying principle.

Methods

A simple model is presented based on the idea of balancing demands for reducing power (derived from photosynthetic electron transport) in primary metabolism versus the secondary pathway that leads to the synthesis of isoprene. This model''s ability to account for key features in a variety of experimental data sets is assessed.

Key results

The model simultaneously predicts the fundamental responses observed in short-term experiments, namely: (1) the decoupling between carbon assimilation and isoprene emission; (2) a continued increase in isoprene emission with photosynthetically active radiation (PAR) at high PAR, after carbon assimilation has saturated; (3) a maximum of isoprene emission at low internal CO₂ concentration (c_i) and an asymptotic decline thereafter with increasing c_i; (4) maintenance of high isoprene emissions when carbon assimilation is restricted by drought; and (5) a temperature optimum higher than that of photosynthesis, but lower than that of isoprene synthase activity.

Conclusions

A simple model was used to test the hypothesis that reducing power available to the synthesis pathway for isoprene varies according to the extent to which the needs of carbon assimilation are satisfied. Despite its simplicity the model explains much in terms of the observed response of isoprene to external drivers as well as the observed decoupling between carbon assimilation and isoprene emission. The concept has the potential to improve global-scale modelling of vegetation isoprene emission.