Modulation of porcine peripheral blood-derived macrophage functions by in vitro infection with African swine fever virus (ASFV) isolates of different virulence

Adaptive Cellular Immunity against African Swine Fever Virus Infections

African swine fever virus (ASFV) remains a threat to global pig populations. Infections with ASFV lead to a hemorrhagic disease with up to 100% lethality in Eurasian domestic and wild pigs. Although myeloid cells are the main target cells for ASFV, T cell responses are impacted by the infection as well. The complex responses remain not well understood, and, consequently, there is no commercially available vaccine. Here, we review the current knowledge about the induction of antiviral T cell responses by cells of the myeloid lineage, as well as T cell responses in infected animals, recent efforts in vaccine research, and T cell epitopes present in ASFV.

Comparative Pathology and Pathogenesis of African Swine Fever Infection in Swine

African Swine Fever (ASF) is a viral disease that affects animals of the Suidae family, and soft ticks from the genus Ornithodoros can also be infected by the ASF virus (ASFV). The disease was first described in Africa at the beginning of the twentieth century as an acute disease characterized by high mortality and fatal hemorrhages. ASF has caused outbreaks in numerous countries and it continues to be devastating nowadays for the porcine sector in those countries affected, and a massive threat for those free of the disease. ASF can follow clinical courses from peracute to chronic in domestic pigs (Sus scrofa) depending on a variety of factors, including the immune status of the animals and the virulence of the ASFV strain. The key features of the pathogenesis of the disease in domestic swine are a) a severe lymphoid depletion including lymphopenia and a state of immunodeficiency, and b) hemorrhages. However, African wild swine like bushpigs (Potamochoerus larvatus), red river hogs (Potamochoerus porcus), and warthogs (Phacochoerus africanus) can be infected by ASFV showing no clinical signs of disease and acting as natural reservoir hosts. In this article we review the key features of the gross and microscopic pathology together with a description of the pathogenesis of ASFV infection in domestic pigs following the different clinical courses. The pathogenesis of ASF in wild and domestic swine is also described, what can provide important information for the design of control strategies, such as vaccines.

Alterations of Nuclear Architecture and Epigenetic Signatures during African Swine Fever Virus Infection

Viral interactions with host nucleus have been thoroughly studied, clarifying molecular mechanisms and providing new antiviral targets. Considering that African swine fever virus (ASFV) intranuclear phase of infection is poorly understood, viral interplay with subnuclear domains and chromatin architecture were addressed. Nuclear speckles, Cajal bodies, and promyelocytic leukaemia nuclear bodies (PML-NBs) were evaluated by immunofluorescence microscopy and Western blot. Further, efficient PML protein knockdown by shRNA lentiviral transduction was used to determine PML-NBs relevance during infection. Nuclear distribution of different histone H3 methylation marks at lysine’s 9, 27 and 36, heterochromatin protein 1 isoforms (HP1α, HPβ and HPγ) and several histone deacetylases (HDACs) were also evaluated to assess chromatin status of the host. Our results reveal morphological disruption of all studied subnuclear domains and severe reduction of viral progeny in PML-knockdown cells. ASFV promotes H3K9me3 and HP1β foci formation from early infection, followed by HP1α and HDAC2 nuclear enrichment, suggesting heterochromatinization of host genome. Finally, closeness between DNA damage response factors, disrupted PML-NBs, and virus-induced heterochromatic regions were identified. In sum, our results demonstrate that ASFV orchestrates spatio-temporal nuclear rearrangements, changing subnuclear domains, relocating Ataxia Telangiectasia Mutated Rad-3 related (ATR)-related factors and promoting heterochromatinization, probably controlling transcription, repressing host gene expression, and favouring viral replication.

Early intranuclear replication of African swine fever virus genome modifies the landscape of the host cell nucleus

Although African swine fever virus (ASFV) replicates in viral cytoplasmic factories, the presence of viral DNA within the host cell nucleus has been previously reported to be essential for productive infection. Herein, we described, for the first time, the intranuclear distribution patterns of viral DNA replication events, preceding those that occur in the cytoplasmic compartment. Using BrdU pulse-labelling experiments, newly synthesized ASFV genomes were exclusively detected inside the host cell nucleus at the early phase of infection, both in swine monocyte-derived macrophages (MDMs) and Vero cells. From 8hpi onwards, BrdU labelling was only observed in ASFV cytoplasmic factories. Our results also show that ASFV specifically activates the Ataxia Telangiectasia Mutated Rad-3 related (ATR) pathway in ASFV-infected swine MDMs from the early phase of infection, most probably because ASFV genome is recognized as foreign DNA. Morphological changes of promyelocytic leukaemia nuclear bodies (PML-NBs), nuclear speckles and Cajal bodies were also found in ASFV-infected swine MDMs, strongly suggesting the viral modulation of cellular antiviral responses and cellular transcription, respectively. As described for other viral infections, the nuclear reorganization that takes place during ASFV infection may also provide an environment that favours its intranuclear replication events. Altogether, our results contribute for a better understanding of ASFV replication strategies, starting with an essential intranuclear DNA replication phase which induces host nucleus changes towards a successful viral infection.

The low-virulent African swine fever virus (ASFV/NH/P68) induces enhanced expression and production of relevant regulatory cytokines (IFNalpha, TNFalpha and IL12p40) on porcine macrophages in comparison to the highly virulent ASFV/L60

The impact of infection by the low-virulent ASFV/NH/P68 (NHV) and the highly virulent ASFV/L60 (L60) isolates on porcine macrophages was assessed through the quantification of IFNα, TNFα, IL12p40, TGFβ and ASFV genes by real-time PCR at 2, 4 and 6 h post-infection. Increased IFNα, TNFα and IL12p40 expression was found in infection with NHV, in which expression of TGFβ was lower than in infection with L60. Principal component analysis showed a positive interaction of cytokines involved in cellular immune mechanisms, namely IFNα and IL12p40 in the NHV infection. Quantification by ELISA confirmed higher production of IFNα, TNFα and IL12p40 in the NHV-infected macrophages. Overall, our studies reinforce and clarify the effect of the NHV infection by targeting cellular and cellular-based immune responses relevant for pig survival against ASFV infection.

The viral protein A238L inhibits TNF-alpha expression through a CBP/p300 transcriptional coactivators pathway

African swine fever virus (ASFV) is able to inhibit TNF-alpha-induced gene expression through the synthesis of A238L protein. This was shown by the use of deletion mutants lacking the A238L gene from the Vero cell-adapted Ba71V ASFV strain and from the virulent isolate E70. To further analyze the molecular mechanism by which the viral gene controls TNF-alpha, we have used Jurkat cells stably transfected with the viral gene to identify the TNF-alpha regulatory elements involved in the induction of the gene after stimulation with PMA and calcium ionophore. We have thus identified the cAMP-responsive element and kappa3 sites on the TNF-alpha promoter as the responsible of the gene activation, and demonstrate that A238L inhibits TNF-alpha expression through these DNA binding sites. This inhibition was partially reverted by overexpression of the transcriptional factors NF-AT, NF-kappaB, and c-Jun. Furthermore, we present evidence that A238L inhibits the activation of TNF-alpha by modulating NF-kappaB, NF-AT, and c-Jun trans activation through a mechanism that involves CREB binding protein/p300 function, because overexpression of these transcriptional coactivators recovers TNF-alpha promoter activity. In addition, we show that A238L is a nuclear protein that binds to the cyclic AMP-responsive element/kappa3 complex, thus displacing the CREB binding protein/p300 coactivators. Taken together, these results establish a novel mechanism in the control of TNF-alpha gene expression by a viral protein that could represent an efficient strategy used by ASFV to evade the innate immune response.

Development of a nested PCR and its internal control for the detection of African swine fever virus (ASFV) in Ornithodoros erraticus

A nested PCR assay, with an internal control, was developed to detect African swine fever virus (ASFV) DNA in Ornithodoros erraticus. The assay revealed a better analytical sensitivity than virus isolation and the OIE PCR protocol. All ticks collected from the field, which were positive by virus isolation, were also positive by PCR. Viral DNA was detected in a further 19 out of 60 ticks from which no virus was isolated. Our results show that this assay is reliable and can easily be used to screen large tick populations collected in the field for the presence of ASFV.

The non-haemadsorbing African swine fever virus isolate ASFV/NH/P68 provides a model for defining the protective anti-virus immune response

African swine fever virus ASFV/NH/P68 is a naturally occurring, non-haemadsorbing and non-fatal isolate. Longitudinal clinical and immunological studies on 31 pigs inoculated oronasally or intramuscularly with this isolate defined two discrete groups of animals: those developing ASF chronic type lesions and those remaining asymptomatic. Animals developing lesions had viraemia and fever late after infection, NK activity levels close to that of control animals and high levels of anti-ASFV specific antibodies together with a marked hypergammaglobulinaemia involving IgG1, IgG2, IgM and IgA immunoglobulin isotypes. Pigs remaining asymptomatic after infection, on the other hand, did not have viraemia or fever after day 14 post-infection and had elevated NK cell activity, but normal plasma Ig concentrations and relatively low specific anti-virus antibody concentrations throughout the duration of the experiments. Importantly, the latter group of pigs virus were resistant to subsequent challenge with the highly virulent ASFV/L60 isolate and survived with no major changes in any of the parameters examined and referred to above. Finally, lymphoproliferative responses to the mitogens concanavalin A, phytohaemagglutinin and pokeweed mitogen were not depressed in either of the two clinically defined groups of pigs. Thus further studies with this infection model may provide new insights on mechanisms of protective immunity to ASFV.

Identification of a 25-aminoacid sequence from the major African swine fever virus structural protein VP72 recognised by porcine cytotoxic T lymphocytes using a lipoprotein based expression system

Identification of African swine fever virus (ASFV) proteins recognised by cytotoxic T lymphocytes (CTL) from swine surviving ASFV/NH/P68 infection was assessed using expression vectors based on the Pseudomonas aeruginosa outer membrane lipoprotein I gene (oprI). Viral antigens expressed as fusion lipoproteins were shown to be taken efficiently by porcine blood-derived macrophages incubated with outer membrane protein preparations from transformed E. coli. To assess recognition by CTL the fusion lipoprotein-treated macrophages were used as targets in 51Cr release microcytotoxicity assays. Using this approach it was shown that the aminoacid sequence HKPHQSKPILTDENDTQRTCSHTNP from the major structural ASFV protein (VP72), encoded by a recombinant clone (pVUB72) is presented by macrophages, which are lysed under restriction of SLA class I antigens. Overall, the results demonstrate that the oprI based vectors are valuable tools to study ASFV-specific CTL activity.

Modulation of T cell and monocyte function in the spleen following infection of pigs with African swine fever virus

Infection of pigs with many strains of African Swine Fever Virus (ASFV) has been shown to cause a loss or marked decrease in the ability of splenocytes to respond to mitogens. These observations have been extended by cell fractionation and reconstitution experiments to show that the mitogen stimulated proliferative capacity of both the CD4+ and CD8+ T cells is affected. Similarly, monocytes which are directly infectable by virus, are functionally defective as antigen presenting cells when added to mitogen stimulated normal T cells. Interestingly, the same T cells which respond poorly in mitogenic assays can be activated by stimulation through the CD3 receptor. In contrast to the defective mitogenic response of T cells, B cell function, as assessed by stimulation through the CD40 ligand in vitro remains intact. There is no evidence for apoptosis in either the T cells or the B cells recovered from the spleens of ASFV infected animals 1-5 days following infection. Although the number of leucocytes which can be recovered from the infected spleen decreases rapidly with progression of the disease, the proportion of the different cell phenotypes remains constant. Thus decreased activity of lymphocytes in lymphoid tissue from ASFV infected animals appears to be directly attributable to infection of the monocytes.

Changes in swine macrophage phenotype after infection with African swine fever virus: cytokine production and responsiveness to interferon-gamma and lipopolysaccharide

Cytokines produced by cells of the immune system, including macrophages, can influence inflammatory responses to viral infection. This has been exploited by viruses, which have developed strategies to direct the immune response towards ineffective responses. African swine fever virus (ASFV) is a double-stranded DNA virus that infects macrophages of domestic swine. In this study, primary cells of monocyte macrophage lineage were obtained from the lungs, peritoneum or blood of domestic swine and, after infection with ASFV, supernatants were tested for cytokines using biological assays. The cytokine transforming growth factor-beta (TGF-beta) was detected after infection of macrophage preparations, but tumour necrosis factor (TNF) and interleukin-1 (IL-1) were not detected. ASFV-infected and uninfected macrophage populations were also tested to assess their ability to respond to cytokines by enhancing production of superoxide in the respiratory burst mechanism. Responses to interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) were suppressed in macrophage populations infected with virus, even at low multiplicities of infection. Addition of TGF-beta to uninfected macrophages resulted in a similar suppression of response, but antibody to TGF-beta did not prevent suppression induced by virus. These results are discussed in relation to the pathology of African swine fever.

Development of new cloning vectors for the production of immunogenic outer membrane fusion proteins in Escherichia coli

The Pseudomonas aeruginosa lipoprotein gene (oprI) was modified by cloning an in-frame polylinker in both orientations at the end of oprI. The resulting plasmids pVUB1 and pVUB2 allow high lipoprotein production in E. coli after IPTG induction. The modified lipoproteins are present in the outer membrane and surface-exposed. Outer membrane-bound fusion proteins of different sizes were produced and used to generate antibodies without use of adjuvant. An 87 bp DNA fragment from the vp72 capsid protein gene of African Swine Fever virus (ASFV) and the entire Leishmania major glycoprotein gp63 gene were expressed in this system. Finally, a fusion lipoprotein containing a 16 amino acid epitope from the pre-S2b region of Hepatitis B virus (HBV) was presented by an antigen-presenting cell line to a T-cell hybridoma while the corresponding cross-linked S2b peptide was not. The results suggest that OprI-based fusion proteins can be used to generate both humoral and cellular immune responses.

Effect of porcine circovirus infection on porcine alveolar macrophage function

The effect of porcine circovirus (PCV) infection of porcine alveolar macrophage cultures on some of the functional properties of these cells are reported. PCV infection of alveolar macrophages did not effect their ability to phagocytose and kill complement-coated yeast cells or the expression of Fc or complement receptors. A transient increase in major histocompatibility complex (MHC) class I expression in PCV-infected cells were observed 4 days after infection and a decrease in the number of cells expressing MHC class II antigens was observed 8 days after infection. Infection of alveolar macrophages with PCV also resulted in a transient decrease in their ability to act as accessory cells in mitogen-induced lymphocyte proliferation of monocyte-depleted porcine peripheral blood mononuclear cells.

Delivery of pseudorabies virus envelope antigens enclosed in immunostimulating complexes (ISCOMs); elicitation of neutralizing antibody and lymphoproliferative responses in swine and protection in mice

An experimental subunit vaccine that consisted of pseudorabies virus (PRV) envelope glycoproteins enclosed into immunostimulating complexes (PRVenv/ISCOM) was constructed, and evaluated in DBA/2 mice and inbred swine of the SLA haplotype c/c. Two to three weeks after the first vaccine dose, specific anti-PRV antibodies could be demonstrated by ELISA or virus neutralization (VN) assays. Booster PRVenv/ISCOM vaccinations resulted in rapid and significant increases in antibody titres in both mice and swine. In addition, a week after receiving the third PRVenv/ISCOM vaccine dose swine peripheral blood mononuclear cells exhibited significant proliferation in response to stimulation with PRV virion antigen. Moreover, two doses of vaccine sufficed to protect mice fully against lethal virus challenge. Therefore, the data presented here support the ISCOM as a viable antigen delivery system for subunit PRV vaccines.

Porcine immune responses to African swine fever virus (ASFV) infection

Immune responses mediating protection against ASFV are poorly understood. Anti-ASFV antibodies may influence the course of the clinical disease but they have never been found to neutralize the virus. Recent developments on cellular defense mechanisms, using swine protection models, and on the induction and role of some cytokines warrant further investigation on these areas.

The relative density of CD44-positive porcine monocytic cell populations varies between isolations and upon culture and influences susceptibility to infection by African swine fever virus

African swine fever (ASF) virus has been reported to infect cells of the monocyte family, probably macrophage-like cells, but there is variation in the apparent susceptibility of these cells. We have demonstrated that the phenotype and activity of porcine monocytic cells varies between different isolations and also upon culture. The variation during culture is dependent upon the phenotype of the cells at the time of isolation. As for the susceptibility of porcine monocytes/macrophages to infection by ASF virus, it was seen that this could be related to the variation in cell phenotype and activity. The susceptibility was determined by the relative density of particular subpopulations of cells present. Whilst inflammatory macrophages did not have an apparent role to play, phagocytic activity was influential. Furthermore, the expression of CD44 and the DH59 myeloid cell marker was important, whereas the relevance of MHC Class II expression was variable. Overall, it was concluded that susceptibility to infection required that a culture be dominated by CD44-positive cells which were non-inflammatory, of low phagocytic activity, and characterizable as being of the myeloid (DH59-positive) lineage.

African swine fever virus specific porcine cytotoxic T cell activity

African swine fever virus (ASFV) specific, cytotoxic T lymphocyte (CTL) activity has been studied in a protection model in which SLA inbred miniature swine are experimentally inoculated with a naturally occurring, non-fatal ASFV isolate (NHV). Peripheral blood mononuclear cells (PBMC) from such infected swine show significant activity in CTL assays, using cultured ASFV-infected porcine blood derived macrophages as target cells. This CTL activity is elicited from PBMC by in vitro restimulation of effector cells with low doses (multiplicity of infection = 0.1) of the homologous virus isolate for 48 to 72 h. For SLAc/c effectors, this CTL activity appears to be SLA class I restricted because (1) blocking target cell antigens with monoclonal antibodies (mAb) against SLA class I antigens causes a major reduction in CTL activity; (2) there is preferential lysis of SLA class I matched, ASFV infected targets; and (3) depletion of effector cells with CD8 specific mAb and complement causes a reduction in CTL activity. The CTL activity is ASFV specific for all pigs tested in that infected macrophages are preferentially lysed as compared to normal (non-infected) cultured macrophages or macrophages infected with hog cholera virus (HCV). Lysis of macrophages infected with different ASFV isolates revealed that there is marked lysis of macrophages infected with the virulent L60 isolate but less lysis of macrophages infected with the DR-II and Tengani isolates. In summary, our data show that ASFV specific CTL activity is triggered in swine infected with the NHV ASFV isolate.

Generation and functional characterization of ovine bone marrow-derived macrophages

A method for the culturing and propagation of ovine bone marrow-derived macrophages (BMM) in vitro is described. Bone marrow cells from sterna of freshly slaughtered sheep were cultured in hydrophobic (teflon foil) bags in the presence of high serum concentrations (20% autologous serum and 20% fetal calf serum). During an 18 day culture period in the absence of added conditioned medium, and without medium change, a strong enrichment of mononuclear phagocytes was achieved. Whereas the number of macrophages increased four to fivefold during this time, granulocytes, lymphoid cells, stem cells and undifferentiated progenitor cells were reduced to less than 3% of their numbers at Day 0. This resulted in BMM populations of 94 +/- 3% purity. These cells had morphological and histochemical characteristics of differentiated macrophages, and they performed functions similar to those of non-activated, unprimed human monocyte-derived macrophages. Thus, they avidly ingested erythrocytes coated with IgG of heterologous or homologous origin. They expressed a modest level of procoagulant activity, but upon triggering with lipopolysaccharide (LPS), a marked increase in cell-associated procoagulant activity was observed. LPS triggering promoted the secretion of interleukin-1, as evidenced by measurement of murine thymocyte costimulatory activity, and transforming growth factor-beta. Using the mouse L929 cell cytotoxicity assay as an indication of tumor necrosis factor (TNF) activity, no TNF activity was detected in the same supernatants, a result possibly due to species restriction. BMM generated low levels of O2- upon triggering with phorbol 12-myristate 13-acetate (PMA). On the other hand, no O2- production was observed upon stimulation with zymosan opsonized with ovine or human serum. Using luminol-enhanced chemiluminescence (CL) as a more sensitive indicator of an oxidative burst, both PMA or zymosan were able to trigger CL, but the response was subject to partial inhibition by sodium azide, an inhibitor of myeloperoxidase. This points to non-macrophage cells contributing also to the CL response, and is consistent with the view that unprimed BMM elicit a low oxidative burst upon triggering with strong inducers of a burst. Our functional characterization now allows us to apply priming and activation protocols and to relate their effect to functional alterations.

Swine leukocyte antigen and macrophage marker expression on both African swine fever virus-infected and non-infected primary porcine macrophage cultures

Swine leukocyte antigens (SLA) and a macrophage specific marker were monitored on porcine macrophages cultured with or without macrophage colony stimulatory factor (M-CSF) and on cells infected with African swine fever virus (ASFV). SLA expression was maximal either in the total cell extract or on the cell surface at 3-4 days of culture; after 4 days these values began to decrease. Fluorescence analyses of immunostained macrophages cultured with or without M-CSF indicated a major upward shift in the number of SLA Class I molecules on individual macrophages whereas for SLA Class II both a novel expression of Class II and an upward shift in the number of molecules per cell were evident. Infection of 3-day-old macrophage cultures with three different isolates of ASFV resulted in minor changes in surface expression of SLA Class I, SLA Class II, and macrophage markers. No differences in infection with ASFV was observed whether macrophages were SLA Class II positive or negative, nor was there blocking by anti-SLA Class I or Class II monoclonal antibodies of ASFV infection of cultured macrophages.

Effect of macrophage-specific colony-stimulating factor (CSF-1) on swine monocyte/macrophage susceptibility to in vitro infection by African swine fever virus

Swine cells of the monocyte/macrophage lineage (MM) proliferate and survive for several weeks in vitro in medium supplemented with the murine macrophage-specific hematopoietic growth factor, colony-stimulating factor 1 (CSF-1). The extent to which MM, cultured in CSF-1, supported African swine fever virus (ASFV) growth in vitro was investigated. MM, cultured in medium with CSF-1, were sensitive to infection and viral-induced cytopathogenic damage by both natural field isolates of ASFV and fibroblast-adapted ASFV strains, as were primary MM (P-MM). Without CSF-1, blood mononuclear leukocytes (MNL), containing lymphocytes and MM, and P-MM could be reliably used in microculture for ASFV titration when inoculated at times limited to no more than 3 to 5 days after culture inception; inclusion of CSF-1 in the media stimulated continued MM survival and growth, and allowed for the use of MNL and P-MM for ASFV titration when inoculated as long as 2 to 3 weeks after microculture inception. MM that were propagated beyond 1 week in secondary culture in medium with CSF-1 (MM-CSF) were useful in microcultures for infective-ASFV titration, only when the cells were kept in medium with CSF-1 and inoculated no later than 3 days of culture inception. In vitro studies of ASFV infection in P-MM and in MM-CSF showed comparable kinetics in ASFV-induced hemadsorption (HAd), cytopathogenic effect (CPE), cytoplasmic viral antigens and nucleic acid material. Compared to P-MM in culture without CSF-1, relatively minor delays in CPE onset induced by some ASFV strains were noticed in MM-CSF and in P-MM that were placed in media with CSF-1. The effects of ASFV on DNA synthesis in the virus-susceptible MM, cultured with or without CSF-1, were also examined at different times of infection by measurement of 3H-thymidine (3H-TdR) incorporation into total precipitable culture material. ASFV-infection of P-MM, placed in culture medium with CSF-1, caused a pronounced transient increase in total 3H-TdR incorporation at the early onset of CPE and HAd. When compared to uninfected P-MM that were stimulated by CSF-1 to synthesize DNA, infected P-MM failed to incorporate 3H-TdR after CPE was fully evident. For P-MM that were cultured without CSF-1 and for MM-CSF, that were kept in culture with CSF-1, transient increases in 3H-TdR incorporation at the onset of CPE and HAd by ASFV-infection were evident, but were much less pronounced.

Inhibition of African swine fever virus in cultured swine monocytes by phosphonoacetic acid (PAA) and by phosphonoformic acid (PFA)

The use of phosphonoacetic (PAA) and phosphonoformic acid (PFA) as inhibitors of African swine fever virus (ASFV) replication in porcine monocytes/macrophages (MO) was investigated. At concentrations sufficient to inhibit replication, hemadsorption, and cytopathogenic damage by high inocula of ASFV, both antiviral agents were cytostatic and suppressed the DNA-synthetic growth response of porcine MO to the MO-specific colony-stimulating factor-1 (CSF-1). PAA and PFA inhibited ASFV-associated DNA-synthesis in the cytoplasm of infected swine MO. Using ASFV-specific monoclonal antibodies in immunebinding assays and in immunoprecipitation analysis of radiolabeled proteins of infected MO, PAA and PFA inhibited the synthesis of ASFV proteins of 13, 73, and 150/220 kDa, and caused a variable inhibition in the synthesis of a 12 kDa ASFV protein. These antiviral drugs, however, did not prevent the appearance of an early 32 kDa ASFV protein. The cytostatic and virus-suppressive effects of PAA and PFA could be reversed. ASFV resumed growth in infected MO cultures, if the cells maintained in medium with CSF-1 were removed from the antivirals before 1 week of drug exposure. With prolonged exposure to PAA or PFA (beyond 1 week), ASFV could not be recovered from infected MO cultures.

In vitro induction of swine peripheral blood monocyte proliferation by the fibroblast-derived murine hematopoietic growth factor CSF-1

The addition of conditioned medium from murine L929 fibroblasts (MGF) to cultures of swine peripheral blood mononuclear cells (MNL) resulted in growth of cells of macrophage/monocyte lineage (MO). Glass-adherent swine MNL, shown to be greater than 95% phagocytic MO, grew in the presence of MGF, whereas swine blood granulocytes and lymphocytes were not MGF-responsive. Primary and secondary MO growth were directly dependent on MGF presence and concentration. MGF-stimulated MO synthesized DNA, as measured by cellular incorporation of tritium-labeled thymidine (3H-TdR). This mitogenic response was maximal by 5 to 6 days in primary MO cultures and declined thereafter to a lower magnitude in secondary MO cultures. In the presence of MGF, viable MO numbers increased with an approximate population doubling time of 5 to 7 days in primary culture. This growth rate was prolonged, to about 10 to 12 days, for MGF-stimulated MO in secondary cultures. MGF removal from primary and secondary MO cultures resulted in rapid growth cessation and cell death. MGF-stimulated MO could not be sustained in secondary culture beyond 7 weeks. MGF-cultured MO were positive for latex phagocytosis, non-specific esterase, Fc-receptor expression, and could mediate antibody-dependent cell-mediated cytotoxicity. The MO-mitogenic principle of MGF was identified as the murine, macrophage-specific colony-stimulating factor, CSF-1 (M-CSF). The swine MO-proliferative response to MGF was inhibited by addition of monospecific goat antisera to M-CSF. Purified M-CSF stimulated the growth of swine MO from cultures of MNL and primary glass-adherent MO.