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Infectious bursal disease is one of the most important naturally occurring viral diseases of chickens worldwide. The causative agent, infectious bursal disease virus (IBDV), belongs to the family Birnaviridae. This virus causes an acute, highly contagious and immunosuppressive disease in chickens. The virus infects and destroys actively dividing IgM-bearing B cells. Although B cells are the principal targets for IBDV, recent data show that the virus also infects macrophages. IBDV-infected macrophages produce various cytokines and chemokines which may play an important role in the protection and/or pathogenesis of IBDV. In this review, the modulatory effects of IBDV on macrophages will be discussed., (Copyright 2007 S. Karger AG, Basel.)

Infectious bursal disease virus (IBDV) infects and replicates in the dividing B lymphocytes of chickens. In the present study, the in vivo effect of IBDV infection on chicken macrophage populations and macrophage activation were examined. Specific-pathogen-free chickens were exposed to virulent IBDV and splenic macrophages were recovered during the acute phase of the disease. At 3 and 5 days post-infection (dpi), spleens of virus-exposed chickens had fewer macrophages than those of virus-free controls (p < 0.05). Confocal microscopic examination revealed cells that were positive for both KUL01 (macrophage surface marker) and R63 (IBDVVP2), indicating presence of the virus in macrophages. MQ-NCSU cells, an avian macrophage cell line, were susceptible to replication of IBDV. In addition, splenic macrophages were activated and had temporarily increased levels of mRNA transcripts of pro-inflammatory mediators, including IL-1beta, IL-6, IL-18, and iNOS. The robust expression of proinflammatory cytokine transcripts, along with a decrease in macrophage numbers, suggest that IBDV activates and may lead to a reduction of resident macrophages in vivo.

Differences in the immunopathogenesis of several strains of infectious bursal disease virus (IBDV) were compared. The strains included a virulent virus (IBDV-IM) and three vaccine viruses that included an intermediate vaccine virus (IBDV-B2) and two mild vaccine viruses (IBDV-Lukert and IBDV-BVM). The most significant differences were found in the systemic effects of these strains. In comparison with other strains, IBDV-IM antigen was detectable for up to 8 days postinfection (PI) in lymphoid tissues that included spleen and cecal tonsils, whereas only a few IBDV-B2- and IBDV-Lukert- and no IBDV-BVM-inoculated birds had detectable IBDV antigen in these tissues. IBDV-IM induced systemic circulating nitrite levels in over 86% of the birds at days 2 and 3 PI. IBDV-IM suppressed most vigorously the splenic mitogenic response on days 3-8 PI. Among the three vaccine strains, IBDV-B2 was the most virulent of the three, inducing a significant suppression of the mitogenic response (P < 0.05) and the most vigorous lesions in the bursa of Fabricius with the highest possible lesion score of 4 at 3 days PI (P < 0.05). IBDV-BVM was the mildest strain, not inducing any detectable lesions in lymphoid tissue at the tested time points. Whereas all IBDV-BVM-inoculated and 67% and 33% of the IBDV-Lukert- and IBDV-B2-inoculated birds, respectively, had detectable IBDV antigen in the bursa at 4 days postchallenge, none of the IBDV-IM-inoculated birds was positive for IBDV by immunohistochemistry. IBDV-IM induced the highest enzyme-linked immunosorbent assay (ELISA) antibody levels detected at days 8-29 PI (P < 0.05) and the best protection against challenge virus replication in comparison with IBDV-B2 and IBDV-Lukert. Only one of five IBDV-BVM-inoculated birds developed anti-IBDV ELISA antibodies at 29 days PI, and none of the birds was protected against IBDV challenge. We speculate that better protection with more virulent strains was due to more systemic antigenic stimulation on the basis of higher replication of IBDV in extrabursal lymphoid tissues. Interestingly, IBDV-IM did not differ from IBDV-B2 and IBDV-Lukert in its ability to induce T cell accumulation in the bursa at 8 days PI and local interferon-gamma induction from days 2 to 5 PI. These results suggested that the local T cell events in the bursa alone may not be indicative of a rapid and protective immune response.

The current belief is that the humoral immune response plays the principal role in defense against virulent infectious bursal disease virus (IBDV). In this study we used a model, in which chickens were compromised in functional T cells by neonatal thymectomy and Cyclosporin A (TxCsA) treatment, to demonstrate the role of T cells in protective immunity against IBDV. We demonstrated that T cells were necessary to achieve full protection against virulent IBDV. When T cell compromised TxCsA-treated chickens were vaccinated with an inactivated IBDV (iIBDV) vaccine, 91% were not protected against IBDV challenge in comparison to T cell-intact chickens, which had a protection rate of 91%. The iIBDV vaccine induced virus neutralizing (VN) and ELISA antibodies, respectively, in 65 and 5% of TxCsA-treated, and in 100 and 58% of T cell-intact birds. These observations provide evidence that the stimulation of T helper cells is needed for the production of protective antibody levels in iIBDV-vaccinated chickens. Passive administration of VN anti-IBDV antibodies inducing a circulating antibody level of log(2)8 in chickens revealed that the levels of antibodies that protected T cell-intact chickens against virulent IBDV challenge were not protective for TxCsA chickens. These results indicated that antibody alone was not adequate in inducing protection against IBDV in chickens and that T cell-involvement was critical for protection. We propose that the inability of iIBDV to protect TxCsA chickens was due to compromised T cell immunity, functional T helper cells and most likely also cytotoxic T cells are needed in iIBDV vaccine protection., (Copyright 2002 Elsevier Science B.V.)

A multivalent in ovo vaccine (MIV) was tested for safety and efficacy in a commercial broiler complex. The MIV comprised five replicating live viruses including serotypes 1, 2, and 3 of Marek's disease virus (MDV), an intermediate infectious bursal disease virus (IBDV) and a recombinant fowl poxvirus (FPV) vector vaccine containing HN and F genes of Newcastle disease virus (NDV). The performance of MIV-vaccinated broilers was compared with that of hatchmates that received turkey herpesvirus (HVT) alone (routinely used in ovo vaccine in the broiler complex). The chickens that hatched from the MIV-injected and HVT-injected eggs were raised under commercial conditions in six barns. Barn 1 housed 17,853 MIV-vaccinated chickens and each of the barns 2-6 housed 18,472-22,798 HVT-vaccinated chickens. The HVT-vaccinated chickens were given infectious bronchitis virus (IBV) and NDV vaccines at hatch and at 2 wk of age. The MIV-vaccinated chickens received IBV vaccine at hatch and IBV + NDV at 2 wk of age. The relative values of hatchability of eggs, livability and weight gain of chickens, and condemnation rates at processing were comparable between the MIV and the HVT groups (P > 0.05). Chickens from the MIV- and the HVT-vaccinated groups were challenged with virulent viruses under laboratory conditions. The resistance of vaccinated chickens against Marek's disease could not be assessed because of high natural resistance of unvaccinated commercial broilers to virulent MDV. The relative resistances of the MIV- and the HVT-vaccinated groups, respectively, against other virulent viruses were as follows: IBDV, 100% for both groups; NDV, 81% vs. 19%; FPV, 86% vs. 0%. The successful use of MIV under field conditions expands the usefulness of the in ovo technology for poultry.

Infectious bursal disease virus (IBDV) induces an acute, highly contagious immunosuppressive disease in young chickens. We examined the role of T cells in IBDV-induced immunopathogenesis and tissue recovery. T cell-intact chickens and birds compromised in their T cell function by a combination of surgical thymectomy and Cyclosporin A treatment (Tx-CsA) were infected with an intermediate vaccine strain of IBDV (Bursine 2, Fort Dodge). Our data revealed that functional T cells were needed to control the IBDV-antigen load in the acute phase of infection at 5 days post infection. The target organ of IBDV, the bursa of Fabricius, of Tx-CsA-birds had a significantly higher antigen load than the one of T cell-intact birds (P < 0.05). Tx-CsA-treatment abrogated the IBDV-induced inflammatory response and significantly (P < 0.05) reduced the incidence of apoptotic bursa cells and the expression of cytokines such as interleukin 2 (IL-2) and interferon-gamma (IFN-gamma) in comparison to T cell-intact birds. T cell-released IL-2 and IFN-gamma may have mediated the induction of inflammation and cell death in T cell-intact birds. The IBDV-induced upregulation of tumor necrosis like-factor (TNF) expression was comparable between T cell-intact and Tx-CsA-birds. Tx-CsA-birds showed a significantly faster resolution of IBDV-induced bursa lesions than T cell-intact birds (P < 0.05). This study suggests that T cells modulate IBDV pathogenesis in two ways: a) they limit viral replication in the bursa in the early phase of the disease at 5 days post infection, and b) intrabursal T cells promote bursal tissue damage and delay tissue recovery possibly through the release of cytokines and cytotoxic effects.

Infectious bursal disease virus (IBDV) is an avian lymphotropic virus that causes immunosuppression. When specific-pathogen-free chickens were exposed to a pathogenic strain of IBDV (IM), the virus rapidly destroyed B cells in the bursa of Fabricius. Extensive viral replication was accompanied by an infiltration of T cells in the bursa. We studied the characteristics of intrabursal T lymphocytes in IBDV-infected chickens and examined whether T cells were involved in virus clearance. Flow cytometric analysis of single-cell suspensions of the bursal tissue revealed that T cells were first detectable at 4 days postinoculation (p.i.). At 7 days p.i., 65% of bursal cells were T cells and 7% were B cells. After virus infection, the numbers of bursal T cells expressing activation markers Ia and CD25 were significantly increased (P<0.03). In addition, IBDV-induced bursal T cells produced elevated levels of interleukin-6-like factor and nitric oxide-inducing factor in vitro. Spleen and bursal cells of IBDV-infected chickens had upregulated gamma interferon gene expression in comparison with virus-free chickens. In IBDV-infected chickens, bursal T cells proliferated in vitro upon stimulation with purified IBDV in a dose-dependent manner (P<0.02), whereas virus-specific T-cell expansion was not detected in the spleen. Cyclosporin A treatment, which reduced the number of circulating T cells and compromised T-cell mitogenesis, increased viral burden in the bursae of IBDV-infected chickens. The results suggest that intrabursal T cells and T-cell-mediated responses may be important in viral clearance and promoting recovery from infection.

We examined the suppressive activity of bursal T cells induced by infectious bursal disease virus (IBDV) in inbred (15x7) and outbred commercial specific-pathogen-free (SPF) chickens. The suppressive activity was measured by the ability of bursal and splenic T cells from IBDV-infected chickens to inhibit mitogenic responses of normal splenocytes. The bursacytes but not the splenocytes of IBDV-infected chickens inhibited the mitogenic responses of normal splenocytes. The mitogenic inhibition by the bursacytes of IBDV-infected chickens was dose-dependent. The suppression was observed both in inbred and non-inbred chickens, and thus, was non MHC-restricted. Cell-sorting experiments revealed that both CD4(+) and CD8(+) cells from the bursa of IBDV-infected chickens, as well as cell-culture supernatants conditioned by these cells, mediated suppression. Suppressor T (Ts) cells may therefore be involved in the immunosuppression induced by IBDV.

Infectious bursal disease virus (IBDV) is an important immunosuppressive virus of chickens. The virus is ubiquitous and, under natural conditions, chickens acquire infection by the oral route. IgM+ cells serve as targets for the virus. The most extensive virus replication takes place in the bursa of Fabricius. The acute phase of the disease lasts for about 7-10 days. Within this phase, bursal follicles are depleted of B cells and the bursa becomes atrophic. Abundant viral antigen can be detected in the bursal follicles and other peripheral lymphoid organs such as the cecal tonsils and spleen. CD4(+) and CD8(+) T cells accumulate at and near the site of virus replication. The virus-induced bursal T cells are activated, exhibit upregulation of cytokine genes, proliferate in response to in vitro stimulation with IBDV and have suppressive properties. Chickens may die during the acute phase of the disease although IBDV induced mortality is highly variable and depends, among other factors, upon the virulence of the virus strain. Chickens that survive the acute disease clear the virus and recover from its pathologic effects. Bursal follicles are repopulated with IgM(+) B cells. Clinical and subclinical infection with IBDV may cause immunosuppression. Both humoral and cellular immune responses are compromised. Inhibition of the humoral immunity is attributed to the destruction of immunoglobulin-producing cells by the virus. Other mechanisms such as altered antigen-presenting and helper T cell functions may also be involved. Infection with IBDV causes a transient inhibition of the in vitro proliferative response of T cells to mitogens. This inhibition is mediated by macrophages which are activated in virus-exposed chickens and exhibit a marked enhancement of expression of a number of cytokine genes. We speculate that T cell cytokines such as interferon (IFN)-gamma may stimulate macrophages to produce nitric oxide (NO) and other cytokines with anti-proliferative activity. Additional studies are needed to identify the possible direct immunosuppressive effect of IBDV on T cells and their functions. Studies are also needed to examine effects of the virus on innate immunity. Earlier data indicate that the virus did not affect normal natural killer (NK) cell levels in chickens.

Objective: To examine effects of virus exposure on embryonic lymphoid organ structure, apoptosis, and lymphoid cell subpopulations., Animals: Eggs of specific pathogen free (SPF) White Leghorn chickens at embryonation day (ED) 17., Procedures: Eggs were inoculated with 2,000 plaque-forming units (PFU) of serotype 1 herpesvirus (Marek's disease virus [MDV 1]), 2,000 PFU of herpesvirus of turkeys (MDV 3), or 1,000 embryo infectious doses (EID50) of infectious bursal disease virus (IBDV). On post-inoculation days (PID) 3 and 5, lymphoid organ to body weight ratios were determined, and bursa of Fabricius, thymus, and spleen were evaluated for lesions and apoptosis. Proportions of lymphoid cell subpopulations of PID-3 chicken embryos and 7- to 10-day-old chicks were quantitated by flow cytometry., Results: Lymphoid organ weights were similar in virus-free, MDV1, and IBDV groups. Embryos inoculated with 2,000 PFU MDV 3/egg had lower bursal weights than virus-free controls. In a repeated trial, MDV 3 (1,000 PFU to 4,000 PFU) did not reduce bursal weights among groups. Histologic changes were seen in bursae after MDV 1 and IBDV inoculation. Apoptosis was greater in bursae of MDV 1-infected embryos than controls. Lymphoid cell subpopulations were similar among all groups with the exception of CD8+ and IgM+ cells in spleens of IBDV-infected 10-day-old chicks., Conclusions and Clinical Relevance: Infection with pathogenic strains of MDV 1 and IBDV did not alter lymphocyte subpopulations in embryos or cause complete destruction of lymphoid organs. Changes in lymphoid cell subpopulations exposed as embryos to IBDV were seen only after hatching.

We studied the long-term effect of infectious bursal disease virus (IBDV) in chickens. Specifically, the restoration of virus-induced bursal lesions and the duration of humoral immunodeficiency were examined. One-week-old specific-pathogen-free chickens were intraocularly inoculated with an intermediate vaccine strain (IBDV-Vac) or a virulent strain (IM-IBDV). At intervals postinoculation (PI), chickens were examined for histopathologic lesions. At 1, 3, 5, 10, or 15 wk PI, the chickens were injected with a mixture of antigens, and primary antibody responses were examined at 10 days postimmunization. Initially, the virus caused extensive necrosis of bursal B lymphocytes. This lesion was accompanied by an infiltration of T lymphocytes. With time, the necrotic lesion in the bursa was resolved. The follicles became partly repopulated with B lymphocytes. The repopulation occurred faster in the chickens exposed to IBDV-Vac than in the chickens exposed to IM-IBDV. By 7 wk PI, 40% and 80% of bursal follicles in IM-IBDV- and IBDV-Vac-inoculated chickens, respectively, were repopulated with immunoglobulin M+ B lymphocytes. Both IBDV-Vac and IM-caused suppression of the primary antibody response to antigens. However, the antibody responses of the chickens exposed to either of the two IBDV strains used were compromised only during the first 6 wk of virus exposure. Subsequently, the antibody response returned to near normal levels.

We used in ovo technology to protect chickens against multiple diseases by inoculating vaccines containing mixtures of live viral agents. A single in ovo injection of a vaccine containing serotypes 1, 2, and 3 of Marek's disease virus (MDV), a vaccine strain of serotype 1 infectious bursal disease virus (IBDV), and recombinant fowl pox vaccine with HN and F genes of Newcastle disease virus (rFP-NDV) induced protection against virulent MDV, IBDV, Newcastle disease virus, and fowl poxvirus. The multiple-agent vaccine induced specific antibodies against the viral agents present in the mixture and did not adversely affect the survival of hatched chickens. Inoculation of a vaccine containing serotypes 1, 2, and 3 of MDV and IBDV did not affect hatchability of eggs, although the addition of rFP-NDV to the mixture reduced hatchability by 23%-26%. In ovo vaccination with a vaccine containing MDV and IBDV vaccine viruses did not exacerbate the inhibitory effect of individual viral agents on humoral and cellular immune competence.

We examined the effects of infectious bursal disease virus (IBDV) on splenic T cells and macrophages. In acute IBDV infection, splenocytes responded poorly to Con A stimulation. However, when T cells were isolated from whole spleen cells, purified T cells responded normally to Con A. This result indicated that functional T cells were present in the spleen but mitogen-induced proliferation of T cells was being suppressed by other cells. Previous studies indicated that soluble factors from suppressor cells may mediate this inhibition of T cell mitogenesis. We thus examined the effects of IBDV on spleen adherent cells. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to quantitate the expression of several cytokine genes in splenic macrophages. In acute IBDV infection, splenic macrophages exhibited enhanced gene expression of type I interferon (IFN), chicken myelomonocytic growth factor (cMGF), an avian homolog of mammalian IL-6, and 9E3/CEF4, an avian homolog of mammalian IL-8. Mitogen-stimulated spleen cell cultures also produced elevated levels of nitric oxide. The elevation of cytokine gene expression by macrophages occurred transiently during the acute phase of viral infection and coincided with in vitro inhibition of T cell mitogenic response of spleen cells.

Groups of specific pathogen-free chickens aged 3 weeks were inoculated with one of three strains of infectious bursal disease virus (IBDV), namely: (1) a classical virulent strain, IM-IBDV; (2) an antigenic variant-E strain, VE-IBDV; and (3) an attenuated vaccine strain, B2-IBDV. The bursa of Fabricius and thymus were examined 1 to 5 days after inoculation for histopathological lesions, IBDV antigen, and in-situ apoptosis. Infection of chickens with IM-, VE-, or B2-IBDV resulted in the appearance of large numbers of apoptotic bursal lymphocytes, both in IBDV antigen-positive and antigen-negative bursal follicles. B2-IBDV induced a slower increase in the IBDV antigen-positive cells and apoptotic cells than did IM- and VE-IBDV. The results suggested that virulent strains and vaccine strains of IBDV enhance endogenous apoptosis of bursal lymphocytes that are free of detectable IBDV antigen. Infection of chickens with IM-IBDV, but not with VE- or B2-IBDV, caused a significant increase in apoptotic cells in the thymic cortical lymphocytes. The apoptotic lymphocytes in the thymus were negative for IBDV antigen. This result indicated that histopathological lesions in the thymus of IM-IBDV-infected chickens may be due to enhanced endogenous apoptosis of thymic lymphocytes without IBDV antigen production in the cells.

Groups of 3-wk-old specific-pathogen-free chickens were inoculated intraocularly with two virulent and one vaccine strain of infectious bursal disease virus (IBDV). During the acute phase of infection, the bursa of Fabricius and cecal tonsil were examined by immunohistochemistry for IBDV antigen and T cell. With all three viruses examined, the infection resulted in the appearance of viral antigen in the bursa accompanied by the presence of CD3+ cells. The CD3+ cells were predominantly TCR2+ and appeared at the site where viral antigens were present. The CD3+ cells continued to persist in the bursa after most of the IgM+ cells and IBDV antigen-positive cells had disappeared. Similar appearance of CD3+ cells was noted following viral antigen accumulation in the germinal centers of cecal tonsils in virulent IBDV-infected chickens. These results demonstrated that infection with IBDV resulted in extensive viral replication in the bursa and the cecal tonsils with an associated accumulation of T cells. The role these T cells may play in the pathogenesis of IBDV infection remains to be established.

Reovirus and infectious bursal disease virus are among the naturally occurring viruses that cause immunosuppression in chickens. Both viruses cause necrotic lesions in the bursa of Fabricius and may destroy B cells. This may explain their ability to cause humoral immune suppression. The mechanism(s) of virus-induced suppression of cellular immunity is not well understood. Both viruses inhibit the mitogenic response of T cells in chickens. We have noted that this inhibition may be mediated by inhibitory cytokines such as transforming growth factor-beta and nitric oxide produced by activated macrophages. Preliminary studies have indicated that pretreatment of chickens with an immunomodulator, acemannan, reduced the reovirus-induced inhibition of T cells.

Lymphocytes obtained from the blood, spleen, and bursa of normal chickens and of chickens infected with infectious bursal disease virus (IBDV) were analyzed for phenotypic expression of CT4, CT8, and immunoglobulin cell surface markers. Single-cell suspensions were stained with monoclonal antibodies by an indirect immunofluorescent assay, and percent staining was quantitated by flow cytometry. Although an appreciable decline from control levels in the percentage of lymphocytes expressing IgM was detected in the spleen and bursa of infected chickens, the relative proportions of lymphocytes expressing CT4 and CT8 in peripheral blood and spleen remained unchanged following infection. These results suggest that whereas humoral immune depression by IBDV may be associated with lysis of antibody-producing B cells, cellular immune depression is not associated with a detectable change in the proportion of helper or cytotoxic/suppressor subpopulations of T lymphocytes.

Specific-pathogen-free (SPF) chickens were exposed to the IM and VA isolates of virulent infectious bursal disease virus (IBDV). Both viruses induced rapidly progressing lymphoid cell depletion in the bursa. The bursal lesions persisted through the observation period of 16 days. The virus-exposed birds also had histologic lesions in the thymus. Thymic lesions peaked at 3-4 days postinoculation (PI) and then subsided. Immunofluorescence (IF) and antigen-capture enzyme-linked immunosorbent assay (ELISA) detected abundant viral antigen in the bursa, but not in the thymus, of chickens during the first week after infection with IM-IBDV or VA-IBDV. This result indicated that the presence of histologic lesions in the thymus was not associated with active infection and replication of the virus in thymic cells. Inoculation of homogenates of bursal and thymic tissues from virus-exposed chickens into embryonated chicken eggs revealed the presence of infectious virus from both tissues. We speculated that the virus recovered from thymus may have been contributed by virus-infected cells that were circulating through the thymus at the time when this tissue was homogenized.

Studies with specific-pathogen-free chickens revealed that chicks hatching from eggs inoculated at the 18th day of embryonation with infectious bursal disease (IBD) vaccine viruses of low virulence (isolates TC-IBDV and BVM-IBDV) developed antibody against IBD virus (IBDV) and resisted challenge with virulent IBDV at 3 weeks of age or older. Embryo vaccination did not adversely affect hatchability of chicks or survival of hatched chicks. Chicks embryonally vaccinated with TC-IBDV had transient histologic lesions in the bursa of Fabricius at hatch. Similar but milder lesions were also noted in chickens that received TC-IBDV at hatch. The level of protection following embryo vaccination with TC-IBDV and BVM-IBDV was similar to that following vaccination with the same vaccines at hatch. Vaccine viruses of moderate virulence (isolates BV-IBDV and 2512-IBDV) were not suitable as vaccines in embryos lacking maternal antibody to IBDV, because the vaccinated chicks developed acute IBD after hatch. Isolate 2512-IBDV was not pathogenic for embryos bearing maternal antibody to IBDV. Maternal antibody against IBDV interfered with efficacy of embryo vaccination with BVM-IBDV but not with 2512-IBDV. Embryo vaccination with a mixture of vaccines against IBD and Marek's disease resulted in protection of hatched chicks against challenge with virulent IBDV and Marek's disease virus.

Specific-pathogen-free chickens inoculated with isolate VA (variant A) or isolate IM of infectious bursal disease virus (IBDV) were examined for mitogenic response to T-cell mitogens, primary and secondary antibody response to sheep erythrocytes and Brucella abortus, and gross and histologic lesions in thymus and bursa. Both isolates induced comparable depression in the mitogenic and antibody response, and both caused extensive gross and histologic lesions in the bursa of Fabricius. However, bursal necrosis induced by the IM isolate was accompanied by an inflammatory response, whereas the inflammatory component was lacking in the lesion induced by the VA isolate. Furthermore, the IM isolate induced extensive lesions in the thymus, but the VA isolate did not.

Spleen cells of chickens infected with IBDV responded poorly to in vitro stimulation with Con A. The mitogenic hyporesponsiveness was due to the presence of suppressor cells that could be removed by pretreatment of IS cells with carbonyl iron or cytodex-3 microcarrier beads. Addition of suppressor cells to normal spleen cells prevented the normal cells from responding to mitogen. Cell-to-cell contact between responder and suppressor cells was not necessary for suppression to occur; the effect was mediated by soluble product(s) released by the suppressor cells. IS cells were also deficient in producing mitogen-induced IL-2 and addition of exogenous IL-2 did not restore mitogenic response of IS.

A pathogenic isolate of infectious bursal disease virus (IBDV) caused persistent and extensive lesions in the bursa but mild and transient lesions in the thymuses of chickens of lines 63 and P. The effect of IBDV on two cellular immune functions, namely, natural killer cell cytotoxicity and mitogenic response, was studied. The natural killer cell activity was not consistently influenced, but the virus, during the first 2 weeks of infection, caused transient depression of the blastogenic response of spleen cells to phytohemagglutinin. Studies on mitogenic hyporesponsiveness revealed that the functional impairment was mediated by a suppressor cell that shared several characteristics with macrophages; i.e., the suppressor cell was adherent to plastic, was phagocytic, and resisted treatment with antithymocyte and antibursa cell sera. Removal of suppressor cells from the spleens of virus-infected chickens resulted in restoration of the mitogenic response of cells. Further, in mixing experiments, the suppressor cell isolated from the spleens of virus-infected chickens also inhibited the mitogenic response of normal spleen cells. We concluded that reduced mitogenic response of lymphocytes in IBDV-infected chickens was not due to a lack of functional T-cells, as suggested previously by others, but was due to macrophage-like suppressor cells. The suppressor cells, although present in certain normal chickens, became activated during early stages of IBDV infection.

Vaccination of specific-pathogen-free chickens as 18-day embryos with the BVM isolate of infectious bursal disease virus (IBDV) resulted in extensive replication of the vaccine virus in the embryonic tissues. The virus was recovered from lung, thymus, proventriculus, liver, kidney, and spleen of embryos 1 day postvaccination, and recoverable virus persisted for at least 7 days. Replication and spread of the vaccine virus in chickens vaccinated as 18-day embryos was compared with that in chickens vaccinated at hatch. Distribution of the virus in tissues was more extensive, virus levels in tissues were generally higher, and detectable virus persisted longer in chickens vaccinated as 18-day embryos than in those vaccinated at hatch. Effective vaccine response could be initiated with 6.2 median embryo lethal doses, the lowest dose tested. Chickens immunized as embryos developed neutralizing antibody against IBDV and resisted challenge with pathogenic IBDV at 4, 6, 8, and 10 weeks of age.

Chickens inoculated at hatch with pathogenic infectious bursal disease virus (IBDV) and the turkey herpesvirus (HVT) vaccine were poorly protected against challenge at 1 week with virulent Marek's disease virus (MDV). However, if vaccination with HVT preceded IBDV inoculation such as by embryonal vaccination with HVT and exposure to IBDV at hatch, or by vaccination at hatch and IBDV exposure 5 days or longer postvaccination, then the vaccinal efficacy of HVT was not influenced. Further, the inhibitory effect of pathogenic IBDV was transient: chickens that received IBDV and HVT at hatch were poorly protected against challenge with virulent MDV at 1 week but not against challenge at 2 weeks of age. Pathogenic IBDV routinely interfered in vaccine efficacy of HVT in chickens lacking maternal antibody to IBDV but not in chickens that had such antibodies at the time of exposure to IBDV. In certain experiments, pathogenic IBDV caused a high rate of mortality in newly hatched chicks. Chickens receiving IBDV and MDV had a lower incidence of Marek's disease than those receiving MDV alone.