Chilean aquaculture and the new challenges: Pathogens, immune response, vaccination and fish diversification

In Chile, the salmon and trout farmed fishing industries have rapidly grown during the last years, becoming one of the most important economic sources for the country. However, infectious diseases caused by bacteria, virus, mycoses and parasites, result in losses of up to 700 million dollars per year for the Chilean aquaculture production with the consequent increase of antibiotic and antiparasitic usage. After 30 years of its first appearance, the main salmon health problem is still the salmonid rickettsial septicaemia (SRS), which together with other disease outbreaks, reveal that vaccines do not provide acceptable levels of long-lasting immune protection in the field. On the other hand, due to the large dependence of the industry on salmonids production, the Chilean government promoted the Aquaculture diversification program by 2009, which includes new species such as Merluccius australis, Cilus gilberti and Genypterus chilensis, however, specific research regarding the immune system and vaccine development are issues that still need to be addressed and must be considered as important as the farm production technologies for new fish species. Based on the experience acquired from the salmonid fish farming, should be mandatory an effort to study the immune system of the new species to develop knowledge for vaccination approaches, aiming to protect these aquaculture species before diseases outbreaks may occur. This review focuses on the current status of the Chilean aquaculture industry, the challenges related to emerging and re-emerging microbial pathogens on salmonid fish farming, and the resulting needs in the development of immune protection by rational designed vaccines. We also discussed about what we have learn from 25 years of salmonid researches and what can be applied to the new Chilean farmed species on immunology and vaccinology.

Time after seawater transfer influences immune cell abundance and responses to SAV3 infection in Atlantic salmon

Pancreas disease (PD) caused by salmonid alphavirus (SAV) severely affects salmonid aquaculture during the seawater phase. To characterize immune cells in target tissues for SAV infection, heart, pancreas and pyloric caeca were analysed from two groups of fish adapted to seawater for 2 and 9 weeks. The sections were scored for the relative abundance of cells expressing MHC class II, IgM, CD3, CD8 or neutrophil/granulocyte markers using immuno-histochemical techniques. In general, necrosis of tissue was more severe in fish infected at 2 weeks post-seawater transfer (wpt) compared with those infected at 9 wpt. At 9 wpt, there were higher numbers of MHC II+ cells in heart, pancreas and pyloric caeca, IgM+ cells in heart and pancreas, and CD3+ cells in pancreas compared to those infected at 2 wpt. The majority of the immune cells infiltrating PD-affected tissues were MHC II+ and CD3+ cells suggesting that antigen-presenting cells and T lymphocytes are the main types of immune cells responding to SAV infection. All the investigated cell types were also observed in pyloric caeca of infected fish, suggesting that this tissue may play a role in the immune response to SAV.

Pseudogymnoascus destructans: evidence of virulent skin invasion for bats under natural conditions, Europe

While Pseudogymnoascus destructans has been responsible for mass bat mortalities from white-nose syndrome (WNS) in North America, its virulence in Europe has been questioned. To shed the light on the issue of host-pathogen interaction between European bats and P. destructans, we examined seventeen bats emerging from the fungus-positive underground hibernacula in the Czech Republic during early spring 2013. Dual wing-membrane biopsies were taken from Barbastella barbastellus (1), Myotis daubentonii (1), Myotis emarginatus (1), Myotis myotis (11), Myotis nattereri (1) and Plecotus auritus (2) for standard histopathology and transmission electron microscopy. Non-lethal collection of suspected WNS lesions was guided by trans-illumination of the wing membranes with ultraviolet light. All bats selected for the present study were PCR-positive for P. destructans and showed microscopic findings consistent with the histopathological criteria for WNS diagnosis. Ultramicroscopy revealed oedema of the connective tissue and derangement of the fibroblasts and elastic fibres associated with skin invasion by P. destructans. Extensive fungal infection induced a marked inflammatory infiltration by neutrophils at the interface between the damaged part of the wing membrane replaced by the fungus and membrane tissue not yet invaded by the pathogen. There was no sign of keratinolytic activity in the stratum corneum. Here, we show that lesions pathognomonic for WNS are common in European bats and may also include overwhelming full-thickness fungal growth through the wing membrane equal in severity to reports from North America. Inter-continental differences in the outcome of WNS in bats in terms of morbidity/mortality may therefore not be due to differences in the pathogen itself.

Establishment of A Competitive ELISA (cELISA) System for the Detection of Influenza A Virus Nucleoprotein Antibodies and its Application to Field Sera from Different Species

A recombinant baculovirus (RBV) encoding the nucleoprotein (NP) of avian influenza virus (AIV) was generated and the appropriate protein was expressed in Sf9 cells. Purified recombinant NP and the NP-specific monoclonal antibody HB65 were used to establish a competitive ELISA (cELISA) system for the detection of NP-specific antibodies in sera of ducks, geese and wild birds. Tests to evaluate this method were carried out using sera of ducks experimentally infected with AIV, pre-immune duck and chicken sera, and poultry field sera, which tested negative in the haemagglutination inhibition (HI) assay, and field sera of several poultry species experimentally infected with other viruses. The evaluation of the test demonstrated a high sensitivity and specificity of this method. Tests carried out using field sera of duck and goose flocks revealed widely corresponding results obtained by HI assay and cELISA indicating that this test is applicable for flock diagnosis. Differing results were obtained for individual samples. It can be assumed that for the most part this was because of a better recognition of the conserved NP antigen by serum antibodies, although some results remained unclear.

Characterization of avian paramyxovirus type 1 strains isolated in Germany during 1992 to 1996

In Germany all avian paramyxoviruses (APMV) isolated in regional laboratories are collected and characterized by the National Reference Laboratory. From 1992 until 1996, 635 APMV-1 virus isolates were submitted from almost all regions. Of these viruses, 371 were isolated from chickens, 39 from other poultry, 171 from pigeons and 54 from exotic birds. All isolates were examined for virulence in intracerebral pathogenicity index (ICPI) tests, for their ability to react with a panel of monoclonal antibodies (mAb) and their thermostability. In addition, the nucleotide sequences of the cleavage site of the fusion protein of a few virus isolates were determined. Most isolates from chickens and other poultry were of the velogenic pathotype. This virus was responsible for the epizootic in 1993 to 1995 in many small flocks. The same virus was obtained from some pigeons and some exotic birds. The pathogenicity of the velogenic/epizootic virus was high with most viruses giving ICPI values of 1.8 to 1.9, and the sequences of the cleavage site of all velogenic isolates tested were closely related. However, viruses isolated at the beginning of the epizootic period differed from viruses isolated towards the end in their reaction with some mAbs. 149 virus isolates were identified as pigeon variant PMV-1 (PPMV-1). Most of these were obtained from pigeons but a few were isolated from chickens and other birds. Most lentogenic isolates proved to be vaccine virus strains.

Potential involvement of rainbow trout thrombocytes in immune functions: a study using a panel of monoclonal antibodies and RT-PCR

The functional relationship between fish and mammalian thrombocytes is relatively unknown. In this study, a panel of monoclonal antibodies (mAbs) was used to investigate the functional properties of rainbow trout thrombocytes. The mAbs recognize cell-surface molecules on thrombocytes with molecular weights ranging from 17 to 160 kDa. Flow cytometric and immuno-electron microscopic analyses demonstrate that these molecules are expressed at different levels and that surface expression increased upon activation with bovine collagen. Two of these cell-surface molecules (17 and 21 kDa) were directly involved in collagen-induced aggregation of thrombocytes since aggregation was blocked upon pre-treatment with mAbs that recognize the two surface markers. Interestingly, the percentage of thrombocytes in blood increased after stimulation using different antigens. The transcriptional profile of trout thrombocytes was then examined after immuno-magnetic enrichment using the described mAbs to assess potential roles of trout thrombocytes in immune functions. Trout thrombocytes express components of the MHC class Ia pathway, IL1beta, TNFalpha, TGFbeta, the interleukin receptor common gamma chain as well as CXC and CC chemokines. MHC class IIB and TNFalpha were expressed at low levels in resting thrombocytes. No evidence was found for the expression of TCRalphabeta, Ig heavy chain, CD8alpha or CK1 mRNA. Taken together, these results suggest that rainbow trout thrombocytes express molecules involved in activation, aggregation and genes encoding proteins, that are involved in antigen presentation and immune regulation.

Recombinant human erythropoietin induces proliferation and Ca 2+ -influx in specific leukocyte subpopulations of rainbow trout ( Oncorhynchus mykiss ) blood and head kidney cells

In mammals, erythropoietin regulates the development and differentiation of erythrocytes. Although hematopoietic cells of bony fish correspond in their ontogeneic development, morphology, and function to their mammalian counterparts, an erythropoietin (EPO)-like molecule has not been identified. In this study we present evidence for a mitogenic response of blood and head kidney leukocytes of rainbow trout after stimulation by recombinant human EPO (rhu EPO). The modulation of cellular activities is accompanied by the induction of DNA-binding activities in nuclear extracts of these cells. In addition, flow cytometric analysis of intracellular Ca2+ concentrations revealed a long-lasting and rhu EPO dose-dependent increase, which was shown to be abrogated by cross-aggregation of surface IgM using anti-trout-IgM monoclonal antibodies (mabs). In flow cytometric dual-labeling experiments using rhu EPO/anti-EPO antiserum and mabs specific for trout leukocyte subpopulations, it was shown that a subpopulation of trout B-cells binds rhu EPO. Moreover, in a modified Ca2+ activation assay, it was demonstrated that this blood B-cell subpopulation is the rhu EPO responder population. In conclusion, the data suggest the existence of EPO-binding receptors in trout that are able to trigger Ca(2+)-independent intracellular signaling in hematopoietic cells of head kidney and Ca(2+)-dependent activation of a subpopulation of B-lymphocytes.

Cattle Infected with Bovine Leukaemia Virus may not only Develop Persistent B-cell Lymphocytosis but also Persistent B-cell Lymphopenia

We investigated the distribution of B and T cells in the peripheral blood of haematologically inconspicuous (non-persistent lymphocytotic, PL-) cattle infected with the bovine leukaemia virus (BLV). Flow cytometric data were obtained from six PL- cattle and compared with six age-matched animals with persistent lymphocytosis (PL+) and five non-infected healthy controls (BLV-). In the PL- group, the percentage and number of surface immunoglobulin-positive (sIg+) B cells were significantly reduced. Whereas in BLV-cattle, about 40% of the peripheral blood lymphocytes (PBL) were sIg + and 24% were sIgM + B cells. In the PL- group, less than 20% of the PBL were sIg+ and sIgM+ B cells. Only 5% of the PBL co-expressed sIgM+ and CD5+ versus 16% in BLV-. This decrease was persistent over 3 years and predominantly affected: (i) B cells that did not express sIgM; (ii) sIgM + B cells co-expressing CD5 and CD11b; and (iii) equally both lambda- and K-type light chain B-cell subpopulations. In contrast, the number of all circulating lymphocytes, CD5- and CD11b- sIgM+ B cells and CD2+ T cells did not differ. In PL+ animals, about 75% of the PBL were sIgM+ CD5+ B cells. These cells were of polyclonal origin, as light chains of the lambda- and K-type were expressed in a ratio of 4:1 (57.7% of PBL lambda+, 14% kappa+) as in BLV- animals (33.6% of PBL lambda+, 8.7% kappa+). In PL+ cattle the absolute number of B-cells and, therefore, their relative percentage is significantly increased. For this reason, even in case of absolutely increased T-cell numbers, the relative percentage of T-cells could be lower than in normal controls. The cause for the observed B cell decrease in PL- cattle is unknown, but it can be assumed that cytotoxic T cells are involved in this B-cell lymphopenia.

Evaluation of immune functions of rainbow trout (Oncorhynchus mykiss)--how can environmental influences be detected?

In fish, the first line of defense against infectious microorganisms is based on a broad range of nonspecific humoral and cellular immune mechanisms ("innate immunity") which without prior specific activation can act in forming a more static barrier (Fish Shellfish Immunol. 10 (2000) 243; Dev. Comp. Immunol. 25 (2001) 827). This natural resistance is normally effective enough to protect fish from infectious diseases until specific immune responses are being induced (Fig. 1; Dev. Comp. Immunol. 25 (2001) 841). Healthy fish exhibit both nonspecific and specific immune responses depending directly on environmental temperature. Pollution of the natural aquatic environment with industrial or agricultural sewage is an important immunosuppressing factor resulting in higher susceptibility to infectious diseases. To date, the possible immunotoxicity of a substance is evaluated using quantification of humoral factors like lysozyme, complement, C-reactive protein or total immunoglobulins but less often using functional assays. Furthermore, most of the functional assays (phagocytosis, respiratory burst, proliferative response) are based on the measurement of the response of resting but not of specific activated immune cells. However, the physiological responses of the immune system to an infection are based on a complex, stepwise activation and proliferation, especially of the specific immune functions after first contact to the microorganisms. In this report we describe in vitro methods for the evaluation of cellular immune functions of different leukocyte populations after specific in vivo triggering of the immune system. Parameters to be evaluated are activation and proliferation of leukocyte populations, phagocytosis and respiratory burst, secretion of antigen-specific antibodies and specific cell-mediated cytotoxicity. Furthermore, challenge models with bacterial (Aeromonas salmonicida) and viral pathogens (Viral Haemorrhagic Septicemia Virus, VHSV) are presented.

Temperature dependent activation of leucocyte populations of rainbow trout, Oncorhynchus mykiss, after intraperitoneal immunisation with Aeromonas salmonicida

The temperature dependence of in vivo activation of rainbow trout Oncorhynchus mykiss, leucocyte populations after intraperitoneal injection (i.p.) of fish with a T-cell independent antigen Aeromonas salmonicida (strain MT423) was investigated using a proliferation assay and flow cytometric analysis with mab specific for trout leucocyte surface markers. In trout kept at 15-17 degrees C a prominent activation of blood and spleen leucocytes was found. Also, drastic changes of the percentage of the leucocyte populations in blood and spleen occurred: the amount of monocytes in the blood increased between day 2 and day 7 post injection (p.i.), whereas in spleen the amount of monocytes stayed at a high level (approximately 35%) after a depression between day 4 and day 7 p.i. The percentage of B-lymphocytes was increased first in spleen and then in blood. The percentage of granulocytes in blood was elevated during the whole experiment compared to control fish. In trout kept at 10-12 degrees C only blood leucocytes showed a weak activation after i.p. injection of A. salmonicida, whereas spleen leucocytes showed nearly no reaction. Only the percentage of granulocytes in the blood (day 2-14 p.i.) and of monocytes in the spleen (day 2 and day 8 p.i.) was changed compared to phosphate buffered saline (PBS)-injected fish. However, the development of A. salmonicida specific antibodies was contrary to the cellular reaction. Whereas antibodies could first be detected after 16-18 days p.i. in both groups the amount of antibodies was significantly higher in sera of trout kept at 10-12 degrees C at day 22 and day 28 p.i. than in sera of trout kept at 15-17 degrees C. These results indicate stronger A. salmonicida induced activation of monocytes, granulocytes and B-lymphocytes at higher temperature. However, the development of a specific antibody response against A. salmonicida seemed to be more effective at lower temperatures.

A monoclonal antibody recognising a surface marker on rainbow trout (Oncorhynchus mykiss) monocytes

The characterisation of a monoclonal antibody (mab 45) reacting with phagocytic leucocytes isolated from blood and spleen of rainbow trout (Oncorhynchus mykiss, Walbaum) is described. The surface marker labelled by this mab is expressed at relative low levels on the membrane of large, nearly nongranulated trout leucocytes, and having the typical morphology of monocytes in flow cytometry (Kfoury et al., 1999, Fish Pathology, 34, 1-6). No reaction of mab 45 with granulocytes, lymphocytes or thrombocytes was detected. In spleen and head kidney, large, polymorphonuclear leucocytes were immunostained. The mab most strongly recognised an antigen of 48 kDa prepared from trout leucocytes of different organs, but not in trout plasma. In an in vitro phagocytosis assay trout monocytes were stained with mab 45 after phagocytosis of Aeromonas salmonicida labelled with the lipophilic fluorescent cell surface linker PKH26. However, previous binding of mab 45 on trout leucocytes did not inhibit the phagocytosis of A. salmonicida particles. Using mab 45, the dynamics of monocytes in blood, spleen and peritoneal cavity could be demonstrated after intraperitoneal injection of trout with inactivated A. salmonicida. The described mab serves as a useful tool to investigate the involvement of monocytes/macrophages in immune reactions of trout to a variety of pathogens.

A new flux-orientated concept to derive critical levels for ozone to protect vegetation

The current European critical levels for ozone (O3) to protect crops, natural and semi-natural vegetation and forest trees are based on a relative small number of open-top chamber experiments with a very limited number of plant species. Therefore, the working group "Effects of Ozone on Plants" of the Commission on Air Pollution Prevention of the Association of German Engineers and the German Institute of Standardization reanalysed the literature on O3 effects on European plant species published between 1989 and 1999. An exposure-response relationship for wild plant species and agricultural crops could be derived from 30 experiments with more than 30 species and 90 data points; the relationship for conifer and deciduous trees is based on 20 experiments with nine species and 50 data points. From these relationships maximum O3 concentrations for different risk stages are deduced, below which the vegetation type is protected on the basis of the respective criteria. Because it is assumed that the fumigation concentrations reflect the O3 concentrations at the top of the canopy, i.e. the upper surface boundary of the quasi-laminar layer if the micrometeorological big-leaf approach is applied, the application of these maximum O3 concentrations requires the transformation of O3 concentrations measured at a reference height above the canopy to the effective phytotoxic concentrations at the top of the canopy. Thus, the approach described in this paper is a synthesis of the classical concept of toxicology of air pollutants (critical concentrations) and the more toxicological relevant dose concept.

Pseudorabies virus glycoprotein K requires the UL20 gene product for processing

Glycoprotein K (gK) of pseudorabies virus (PrV) has recently been identified as a virion component which is dispensable for viral entry but required for direct cell-to-cell spread. Electron microscopic data suggested a possible function of gK in virus egress by preventing immediate fusion of released virus particles with the plasma membrane (B. G. Klupp, J. Baumeister, P. Dietz, H. Granzow, and T. C. Mettenleiter, J. Virol. 72:1949-1958, 1998). For more detailed analysis, a PrV mutant with a deletion of the UL53 (gK) open reading frame (ORF) from codons 48 to 275 was constructed, and the protein was analyzed with two monoclonal antibodies directed against PrV gK. The salient findings of this report are as follows. (i) From the PrV UL53 ORF, a functional gK is translated only from the first in-frame methionine. From the second in-frame methionine, a nonfunctional product is expressed which is not incorporated into virions. (ii) When constitutively expressed in a stable cell line without other viral proteins, gK is only incompletely processed. After superinfection with gK-deletion mutants, proper processing is restored and mature gK is incorporated into virions. (iii) The UL20 gene product is specifically required for processing of gK. gK is not correctly processed in a UL20 deletion mutant of PrV, and superinfection of gK-expressing cells with PrV-UL20(-) does not restore processing. However, all other known structural viral glycoproteins appear to be processed normally in PrV-UL20(-)-infected cells. (iv) Coexpression of gK and UL20 restored gK processing at least partially. Thus, our data show that the UL20 gene product is required for proper processing of PrV gK.

Intranuclear inclusions in cells infected with Newcastle disease virus

Cells infected by Newcastle Disease Virus were observed to contain both intracytoplasmic and intranuclear inclusion bodies. Ultrastructurally, they consisted of twisted strands of about 18-20 nm diameter resembling nucleocapsids. The presence of these inclusions was detected irrespective of host cell or pathogenicity of the virus. In immunofluorescence and immunogold labelling experiments, these structures were tagged by an anti-P protein monoclonal antibody. In summary, we show that intracytoplasmic and intranuclear inclusion bodies, hitherto used as a taxonomic characteristic for the genus Morbillivirus of the Paramyxoviridae, also occur in a member of the genus Rubulavirus.

Pathogenic, antigenic and molecular properties of rabbit haemorrhagic disease virus (RHDV) isolated from vaccinated rabbits: detection and characterization of antigenic variants

Rabbit haemorrhagic disease virus (RHDV) isolates were obtained from several animals previously vaccinated with an inactivated vaccine. Seven isolates were analyzed by immunological and molecular biological methods and compared to reference strains. Antigenic characterization with monoclonal antibodies as well as haemagglutination assays demonstrated considerable differences between individual isolates. However, sequencing of the capsid protein genes revealed a high degree of homology between five of these isolates and the reference strain FRG. In contrast, two isolates specified remarkably different capsid proteins with a degree of variation not observed so far in RHDV. Amino acid alterations were found clustered between residues 301 and 328 (region C), 344 and 434 (region E) and also in the 3' region of the capsid protein gene. Interestingly, experimental vaccination of rabbits followed by challenge with the heterologous variant strains showed restricted cross-protection against one of the strains. In summary, we found a level of antigenic variation not detected in RHDV so far, and describe two distinct new antigenic variants.

A second form of infectious bursal disease virus-associated tubule contains VP4

Preparations of density gradient-purified infectious bursal disease virus (IBDV) were found to contain full and empty icosahedral virions, type I tubules with a diameter of about 60 nm, and type II tubules 24 to 26 nm in diameter. By immunoelectron microscopy we demonstrate that virions and both types of tubular structures specifically react with anti-IBDV serum. In infected cells intracytoplasmic and intranuclear type II tubules reacted exclusively with an anti-VP4 monoclonal antibody, as did type II tubules in virion preparations. The immunofluorescence pattern with the anti-VP4 antibody correlated with electron microscopical findings. Neither purified extracellular nor intracellular virions were labeled with the anti-VP4 MAb. Our data show that the type II tubules contain VP4 and suggest that VP4 is not part of the virus particle.

Physiological changes on agricultural crops induced by different ambient ozone exposure regimes: I. Effects on photosynthesis and assimilate allocation in spring wheat

Spring wheat (Triticum aeslivum cv. Nandu) cultivated under glasshouse conditions was exposed to ozone in large fumigation chambers for 2 wk. Different exposure regimes were applied as constant concentrations as well as with ozone peaks, partly under equal dose-conditions, in times of high solar radiation during different stages of development (seedling, late tillering, anthesis). Chlorophyll fluorescence was monitored and amounts of carbohydrates (hexoses, sucrose, starch) and chlorophyll were measured in young leaves (seedling) and flag leaves (late tillering, anthesis) during and after ozone exposure. Although seedlings showed no significant response in photosynthesis, strong effects on photosynthesis and carbohydrate accumulation were measured when plants were fumigated during anthesis, especially after a heat stress period preceding ozone treatments. Under equal dose conditions chlorophyll fluorescence parameters (F_v :F_m ) and electron transport rate decreased and sucrose content of flag leaves increased significantly if ozone at a concentration of 220 μg m^-3 was supplied for 4 h, indicating that peak concentrations show stronger effects than constant concentrations. The reaction of wheat plants is dependent on environmental conditions such as preceding heat stress and on the developmental stage during exposure. The results favour the hypothesis that photoinhibition and disturbance of photosynthesis are only secondary effects as a consequence of retarded sucrose export from the leaf, because of damage at the plasma membrane.

VP5 of infectious bursal disease virus is not essential for viral replication in cell culture

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, encodes in its bisegmented double-stranded RNA genome four structural virion proteins, VP1, VP2, VP3, and VP4, as well as a nonstructural protein, VP5. Recently, the establishment of an infectious cRNA system for IBDV has been described (E. Mundt and V. N. Vakharia, Proc. Natl. Acad. Sci. USA 93:11131-11136, 1996). Here, we report the isolation of a VP5- IBDV mutant constructed by site-directed mutagenesis of the methionine start codon of VP5, followed by cRNA transfection. The resulting virus mutant was replication competent in cell culture, which indicates that VP5 is not required for productive replication of IBDV. Absence of VP5 expression was verified by lack of reactivity with newly established anti-VP5 monoclonal antibodies and polyclonal sera. VP5- IBDV exhibited a delay in replication in chicken embryo cells compared to the VP5+ parental virus. However, final yields were similar. Our results thus show that VP5 is nonessential for IBDV replication, which makes it a prime candidate for the construction of deleted, marked vaccines.

Physiological and biochemical characterization of glyoxalase I, a general marker for cell proliferation, from a soybean cell suspension

Using a strictly auxin-dependent soybean (Glycine max (L.) Merr.) cell suspension, we studied the correlation of auxin-dependent cell proliferation and the activity of glyoxalase I (S-lactoylglutathione-lyase EC 4.4.1.5), and enzyme generally associated with cell proliferation in animal, microbial and, as reported recently, also plant systems. We found the activity of glyoxalase I to be modulated during the proliferation cycle, with a maximal activity between day 2 and day 4 of culture growth. After starving the culture of auxins for three subsequent periods, both the enzyme activity and cell-growth could be re-initiated with auxin. Enzyme activity reached its maximum 1 d before cell number was at a maximum. The enzyme was purified to homogeneity and characterized.