Effects of ultraviolet-inactivated feline leukemia virus on the production of alpha/beta interferon by feline peripheral blood mononuclear cells.

The effects of ultraviolet-inactivated feline leukemia virus (UV-FeLV) on the production of alpha and/or beta interferon (IFN) by mononuclear cells stimulated with the Newcastle disease virus (IFN-NDV) was investigated. The production of IFN-NDV and gamma IFN was suppressed by 50% as compared to the control in the presence of 200 ng/ml or 200 micrograms/ml of UV-FeLV, respectively. The presence of UV-FeLV decreased the rate of the production, but the time to reach the plateau of the IFN activity in the culture supernatant was not shortened in both IFNs. It was suggested that the suppressive effects of UV-FeLV against both IFNs were through rather similar mechanisms. The suppressive effects of UV-FeLV on IFN-NDV production was most evident in the cultures to which UV-FeLV was added at the early stages of the culture, and could not be demonstrated when UV-FeLV was added 8 hr after initiation of the culture. These data therefore indicate that non-infectious feline leukemia viral particles can modify the production of feline IFNs by peripheral blood mononuclear cells.

Lymphocytotoxic strains of feline leukemia virus induce apoptosis in feline T4-thymic lymphoma cells.

Feline leukemia retrovirus (FeLV) strains with subgroup C env genes kill feline T4 lymphoma 3201 cells by 7 to 12 days after in vitro inoculation, whereas FeLV strains with subgroup A env genes do not. Neither FeLV-A nor FeLV-C kill feline fibroblasts. FeLV-C, but not FeLV-A, is replicated to higher titer by 3201 cells and productive infection precedes death by 3 to 7 days. Transcriptional activity of the FeLV-C long terminal repeat, as assessed by chloramphenicol acetyltransferase activity, is high in feline lymphoid cells but low in feline fibroblasts. Activity of the FeLV-A long terminal repeat is moderate in both cell types. FeLV-C-infected cells form aggregates 1 to 4 days before dying; ultrastructurally, virus particles can be seen approximating the clustered cells. Dying cells demonstrate nuclear condensation, surface blebbing, and fragmentation. DNA fragmentation and laddering compatible with apoptosis occur 1 to 2 days before massive cell death. In FeLV-C-infected 3201 cells, a shift from phospholipid to neutral lipid incorporation of [14C]oleic acid, increases in palmitic acid proportions and decreases in linoleic acid proportions occur 1 to 2 days before peak killing. Exposure of 3201 cells to ultraviolet-inactivated FeLV-KT (200-800 micrograms/10(6) cells) causes cytostasis within 2 days and death within 4 days. Blebbing and nuclear condensation occur but clusters do not form. The induction of programmed cell death in feline thymic lymphoma cells by subgroup C feline retroviruses may be relevant to the pathogenesis of FeLV-induced thymic atrophy, paracortical lymphoid depletion and acquired immunodeficiency in vivo.

Photodynamic inactivation of retrovirus by benzoporphyrin derivative: a feline leukemia virus model.

The ability of a photosensitizer, benzoporphyrin derivative monoacid ring A (BPD-MA), and either broad-spectrum (400-1200 nm) or narrow-band (600-700 nm) red light to kill feline leukemia virus (FeLV) and FeLV-infected cat T cells (cell line 3201) was investigated in culture medium containing fetal calf serum and in blood from infected cats. A molecular clone of FeLV, 61E, is minimally pathogenic and productively infects 3201 cells while causing no change in rate of cell division, viability, or size. Active virus (either free or within infected cells) was quantified by using a limiting dilution assay that involved cocultivation of test samples with naive 3201 cells, after which either the polymerase chain reaction or a reverse transcriptase assay was used to detect the presence of virus. It was shown that 61E-infected T cells in culture were slightly more sensitive to photodynamic killing than were uninfected cells. Infected cells and free virus were eliminated from whole blood taken from infected cats by using 4 micrograms per mL of BPD-MA and 40 J per cm2 of red light. These results correlate well with previous results with BPD-MA and vesicular stomatitis virus in whole human blood and suggest that this photosensitizer is a promising agent for the elimination of retroviruses that are either free or located within infected cells in blood.

Viral inactivation in blood and red cell concentrates with benzoporphyrin derivative.

Using both the vesicular stomatitis virus (VSV) and feline leukemia virus (FeLV) as models we have shown that the photosensitizer benzoporphyrin derivative ring A (BPD), when activated with red light (600-700 nm), is effective in eliminating both free virus and virally infected cells from spiked blood products and whole blood drawn from viremic cats experimentally infected with FeLV, under conditions which appear to share red blood cells. The effect of photodynamic therapy on infected lymphocytes, as visualized by scanning electron microscopy, initially appeared as a limited area of tiny holes in the membrane. These holes were subsequently seen to increase in size until the membrane appeared completely decomposed. The red cell membranes however, seem to be undamaged by such photodynamic treatment.

Inhibition of phorbol ester-induced neutrophil chemiluminescence by FeLV.

Receptor-mediated activation is accompanied by phospholipid metabolism and by calcium fluctuation resulting in a chemiluminescence (CL) response in the neutrophil. This pathway involves activation of protein kinase C (PKC) and the NADPH oxidase. Artificial stimulants such as phorbol esters, specifically 12-O-tetradecanylphorbol-13-acetate (TPA), circumvent the receptor-mediated pathway and activate PKC resulting in a measurable CL response. Neutrophils from feline leukemia virus (FeLV) exposed cats were tested for their ability to generate a TPA-induced CL response. As compared to the non-FeLV-exposed specific-pathogen-free (SPF) control cat neutrophil CL responses, both viremic and nonviremic FeLV-exposed cats showed significant decreases in their CL responsiveness. Neither ultraviolet light-inactivated FeLV (UV-FeLV) nor protein components (FeLV-p15E and FeLV-p27) caused a significant decrease in the CL responses of the SPF cat neutrophils. The suppressed TPA-induced CL response from FeLV-infected cats may involve an intracellular mechanism not affected in vitro by exposure of the neutrophil to the virus or viral components.

Influence of inactivated feline retrovirus on feline alpha interferon and immunoglobulin production.

The effect of ultraviolet-inactivated feline leukaemia virus (UV-FeLV) on the development of feline immunoglobulin-secreting cells (ISC) was investigated using a reverse haemolytic plaque assay. Low concentrations of UV-FeLV at 2 X 10(-4) to 2.0 micrograms/ml stimulated the production of ISC. By contrast, the same concentration of UV-FeLV suppressed the development of pokeweed mitogen (PWM)-driven ISC. Maximum suppression of ISC occurred at 50 micrograms/ml of UV-FeLV. The generation of an interferon resistant to acid, heat and sodium dodecyl sulphate (SDS) in the media of lymphocyte cultures incubated with PWM was also significantly suppressed in the presence of 0.2 microgram/ml of UV-FeLV. These findings suggest that non-infectious viral particles appear to modify feline immunoglobulin and interferon-secreting systems.

Suppression of in vitro neutrophil function by feline leukaemia virus (FeLV) and purified FeLV-p15E.

Feline neutrophils (PMN) were isolated and exposed to ultraviolet light-inactivated feline leukaemia virus (UV-FeLV) and purified envelope component p15E (FeLV-p15E). Functional capacity of exposed PMN was measured in vitro utilizing the chemiluminescence (CL) response. PMN exposed to UV-FeLV demonstrated depressed CL responses to Ca2+-ionophore A23187 and latex particles. However, FeLV-p15E produced significant suppression in the CL response to A23187 but failed to produce significant alterations in response to latex particles. The data indicate that FeLV-p15E may, in part, be responsible for increased morbidity and mortality among FeLV-infected cats through suppression of the PMN population.

Suppressive effect on polyclonal B-cell activation of a synthetic peptide homologous to a transmembrane component of oncogenic retroviruses.

Purified feline leukemia virus, UV light-inactivated feline leukemia virus, and a synthetic peptide (CKS-17) homologous to a well-conserved region of the transmembrane components of several human and animal retroviruses were each studied for their effects on IgG production by feline peripheral blood lymphocytes. Using a reverse hemolytic plaque assay, both the viable virus and the UV-inactivated feline leukemia virus, but not the CKS-17, activated B lymphocytes to secrete IgG. When staphylococcal protein A, a polyclonal B-cell activator, was used to stimulate IgG synthesis by feline lymphocytes, the viable virus, the UV-inactivated virus, and the CKS-17 peptide each strongly suppressed IgG secretion without compromising viability of the lymphocytes. These findings suggest that the immunosuppressive influences of feline leukemia virus on immunoglobulin synthesis may reside in a conserved portion of the envelope glycoprotein that includes the region homologous to CKS-17.

Polymorphonuclear leukocyte dysfunction associated with feline leukaemia virus infection.

The chemiluminescent characteristics of enriched (greater than 95%) peripheral blood polymorphonuclear leukocyte populations (PMN) from normal and feline leukaemia virus (FeLV)-infected cats were investigated. FeLV-infected cats demonstrated a significantly lower (P less than 0.001) PMN chemiluminescent response when compared to the response of normal age-matched controls. Normal PMN treated with FeLV-infected cat serum exhibited a depressed response in comparison to control cells. A titration of serum from infected cats supplemented with normal serum revealed a titratable suppression of chemiluminescence with increasing concentration of serum from the infected cats. However, PMN from FeLV-infected cats treated with normal serum displayed a slight increase in chemiluminescence over the same cells in autologous serum. The addition of inactivated FeLV to normal PMN caused a titratable decrease in chemiluminescence.

Retrovirus-mediated immunosuppression. II. FeLV-UV alters in vitro murine T lymphocyte behavior by reversibly impairing lymphokine secretion.

Murine splenocytes and cloned murine T cells were used to study the in vitro immunosuppressive effects of UV-inactivated feline leukemia virus (FeLV-UV) on lymphokine secretion. FeLV-UV can significantly depress the accumulation of IL 2 in cultures of Con A-stimulated C57BL/6 splenocytes and in cultures containing the alloreactive helper T cell clone B6D/2-2m plus Con A. Inhibition of lymphokine accumulation in these cultures could not be attributed to absorption or inactivation of IL 2 by the FeLV-UV or to the FeLV-UV-induced production of substances which interfere with the IL 2 bioassay. Thus, FeLV-UV appears to block production and/or secretion of IL 2 by a direct inhibitory effect on IL 2-secreting murine T lymphocytes. Additional studies indicate that FeLV-UV impairs IL 2 production only if added very soon after lymphocyte contact with lymphokine-inducing agents and that IL 2 secretion resumes when FeLV-UV is removed from the culture. FeLV-UV also impairs accumulation of MAF (interferon-gamma?) in cultures of Con A-stimulated C57BL/6 splenocytes and in cultures containing the alloreactive cytotoxic T lymphocyte clone B6D/2-7c plus Con A. The latter observation again suggests that FeLV-UV impairs lymphokine secretion by a direct effect on lymphokine-producing T lymphocytes. Furthermore, it suggests that FeLV-UV does not selectively impair production of IL 2 nor does it have selective inhibitory effects on helper T cells. Rather, FeLV-UV appears to have a general inhibitory effect on lymphokine production by T lymphocytes. Finally, concentrations of FeLV-UV which suppress MAF production by the CTL clone have little influence on the cytolysis mediated by the same cloned T cell population. Thus, the immunosuppressive influence of FeLV-UV is selective for phenomena associated with induction of new T lymphocyte functions, such as lymphokine secretion, and spares other immune functions already expressed by the same cells.

Retrovirus-mediated immunosuppression. I. FeLV-UV and specific FeLV proteins alter T lymphocyte behavior by inducing hyporesponsiveness to lymphokines.

Murine splenocytes were used to study the in vitro immunosuppressive effects of UV-inactivated feline leukemia virus (FeLV-UV). FeLV-UV blocks both alloantigen (DBA/2)-induced and Con A-induced proliferation of C57BL/6 splenocytes in a dose-dependent manner. Furthermore, C57BL/6 anti-DBA/2 mixed lymphocyte cultures containing FeLV-UV fail to develop detectable DBA/2-specific cytolytic activity, although FeLV-UV has no effect on the cytolytic activity of preformed C57BL/6 anti-DBA/2 cytolytic T cells (CTL). Disruption of lymphocyte proliferation and CTL generation by FeLV-UV could not be overcome by the addition of exogenous lymphokines. These data suggest that FeLV-UV can interfere with the lymphokine reactivity of alloactivated lymphocytes. In fact, FeLV-UV blocks the lymphokine-induced proliferation of the murine IL 2-dependent cell line CTLL-20. The CTLL-20 cells were subsequently used to study the mechanism(s) by which retroviruses alter T lymphocyte function. Normally, CTLL-20 cells undergo significant proliferation when cultured in EL4 SN, an IL 2-containing culture supernatant from PMA-stimulated EL4 cells. This lymphokine-induced CTLL-20 proliferation is abrogated in a dose-dependent manner by UV-inactivated murine leukemia virus (MuLV-UV), FeLV-UV, and a purified 15,000 dalton viral protein, p15, derived from FeLV. Suppression of CTLL-20 proliferation requires only brief contact (6 hr) with FeLV-UV or with p15, but is most efficient after prolonged (24 hr) contact with these agents. Furthermore, suppression of CTLL-20 proliferation by FeLV-UV and p15 is reversible, because CTLL-20 cells which have been pretreated for 24 hr with FeLV-UV or p15 are equally as efficient at responding to EL4 SN as untreated CTLL-20. Additional studies indicate that CTLL-20 cells continue to remove IL 2 activity from EL4 SN in the presence of suppressive concentrations of FeLV-UV, and that suppressive concentrations of FeLV-UV do not remove IL 2 activity from EL4 SN. This suggests that FeLV does not block CTLL-20 proliferation by absorbing or inactivating IL 2, or by occluding IL 2 receptors, and that T lymphocytes develop an insensitivity to lymphokines after contact with FeLV-UV, which may be caused by a metabolic, rather than an immunologic, defect. Because lymphokines are requisite signals for T cell function, considerable immunosuppression would be associated with acquired lymphokine insensitivity.

Inhibition of erythroid colony-forming cells by a Mr 15,000 protein of feline leukemia virus.

The effects of concentrated ultraviolet-inactivated feline leukemia virus (FeLV), the purified Mr 15,000 envelope protein (p15E) of FeLV, or the purified Mr 27,000 structural protein (p27) of FeLV on feline bone marrow mononuclear cells were studied in vitro in methylcellulose cultures. Whole virus and purified viral proteins were from the Kawakami-Theilen isolate of FeLV, which induces erythroid aplasia in cats. Bone marrow mononuclear cells from FeLV-negative young adult cats were preincubated with a medium control, ultraviolet-inactivated whole virus, or the p15E or p27 of FeLV, incubated in methylcellulose cultures for 2 days, and then observed for the formation of colony-forming units-erythroid (CFU-E) and colony-forming units-granulocyte/macrophage. The ultraviolet-inactivated Kawakami-Theilen isolate of FeLV at concentrations of 10 or 20 micrograms of viral protein/5 X 10(4) cells suppressed CFU-E to 66 to 56% of control values but had no significant effect on proliferation of colony-forming units-granulocyte/macrophage. p15E at concentrations of 0.1 to 0.2 micrograms/5 X 10(4) cells decreased CFU-E numbers to 0 to 1% of control values, whereas the same concentration of p27 did not alter CFU-E growth when compared with controls. Neither p15E nor p27 had a significant effect on growth of colony-forming units-granulocyte/macrophage. The erythrosuppressive effects of whole virus and an envelope-derived protein but not a structural core protein suggest that FeLV envelope proteins are important in the selective inhibition of erythrogenesis observed in vivo in FeLV-infected cats.

The mechanism of retrovirus suppression of human T cell proliferation in vitro.

Immunosuppression is commonly associated with retrovirus-induced animal tumors. Studies in the murine and feline retrovirus systems suggest that the 15,000-dalton envelope protein (p15E) of the virion may contribute to immunosuppression by interfering with normal lymphocyte function. We examined the effect of inactivated feline leukemia virus (UV-FeLV) and p15E derived from this virus on concanavalin A (Con A) driven human T cell proliferation. Virus and p15E markedly suppressed mononuclear cell proliferative response to Con A. Suppression was not due to inhibition of monocyte accessory cell function, or interleukin 1 (IL 1) secretion. In fact, the presence of monocytes partially protected T cells from UV-FeLV suppression. UV-FeLV, however, suppressed T cell secretion of and response to interleukin 2 (IL 2). We conclude that UV-FeLV and derived p15E inhibit T cell proliferation by direct inhibition of T cell function. These findings, extended to the in vivo situations, suggest that retrovirus-associated suppression of the immune response involves the induction of T cell but not monocyte dysfunction.

Inactivation of tumor cell-associated feline oncornavirus for preparation of an infectious virus-free tumor cell immunogen.

Ultraviolet (UV) and thermal methods of inactivating the oncogenic potential of C-type particle-producing feline oncornavirus-induced tumor cells were developed. The techniques were evaluated by several parameters for their use in preparation of cellular immunogens. The UV inactivation dose required to reduce the number of focus-forming units per ml by 1 log10 for FL-74 lymphoblastoid cell-associated feline leukemia virus was 44,000 ergs/sq mm, and the thermal inactivation dose required to reduce the number of focus-forming units per ml by 1 log10 at 45 degrees was 16 min. Inactivation of greater than 6 log10 of virus per ml associated with 4 x 10(8) cells required a UV dose of 270,000 ergs/sq mm, 100 min at 45 degrees or 3 min at 56 degrees. All three treatments concomitantly destroyed the replicating potential of FL-74 cells as shown by their inability to propagate under normal growth conditions and to incorporate [3H]thymidine into nuclear DNA. UV inactivation and thermal inactivation at 45 degrees allowed the best retention of feline oncornavirus-associated cell membrane antigen. A 50% loss in antigenic activity was observed as a result of 56 degrees treatment, but this method was the only one that did not destroy the surface structural integrity of FL-74 cells.