The role of IL-5, IL-6 and IL-10 in primary and vaccine-primed immune responses to infection with Friend retrovirus (Murine leukaemia virus)

Beatrice D. Strestik1, Anke R. M. Olbrich1, Kim J. Hasenkrug2 and Ulf Dittmer1

Institut für Virologie der Universität Würzburg, Versbacher Str.7, 97078 Würzburg, Germany1
Laboratory of Persistent Viral Diseases, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, MT 59840, Hamilton, USA2

Author for correspondence: Ulf Dittmer. Fax +49 931 2013934. e-mail ulf.dittmer{at}mail.uni-wuerzburg.de


   Abstract
Top
Abstract
Main text
References
 
The defence of a host against viral infections is strongly influenced by cytokines. We investigated the role of the B-cell stimulating cytokines IL-5 and IL-6, and the immuno-suppressive cytokine IL-10, during primary and secondary immune responses in mice against infection with Friend retrovirus (FV) (Murine leukaemia virus). IL-5-/- mice were comparable to C57BL/6 wild-type mice in their ability to control acute FV infection. In contrast, IL-6-/- and IL-10-/- mice showed significantly enhanced virus loads in spleen cells. However, this impaired control of acute FV replication did not alter the long-term control over persistent FV in IL-6-/- and IL-10-/- mice. Immunization with a live attenuated vaccine virus prior to challenge protected all three types of cytokine-deficient mice from high levels of spleen virus, despite the finding that the vaccinated IL-5- and IL-6-deficient mice had significantly reduced titres of virus-neutralizing IgG class antibodies. The results indicate that IL-6 and IL-10 contribute to primary immune responses against FV, but are dispensable during persistent infection and vaccine-primed secondary responses.


   Main text
Top
Abstract
Main text
References
 
Cytokines are the most important regulators of immune responses against foreign antigens. The cytokine profile of an organism in response to an infectious agent is mainly regulated by CD4+ T-helper cells (Th), which can be subdivided into the distinct subsets Th1 and Th2 based on the cytokines they produce (Constant & Bottomly, 1997 ). We have previously used cytokine-deficient mice infected with Friend virus (FV) (Murine leukaemia virus) to study the significance of Th1 and Th2 cells in the immune response against retroviral infections (Dittmer et al., 2001 ). The cytokines analysed in that particular study were the key cytokines for Th1 versus Th2 responses, namely IL-4, IL-12 and IFN-{gamma}. However, there are other important cytokines that also regulate specific functions of the immune system and might therefore contribute to immunity against retroviral infections. Since antibody-secreting B-cells are critical for a protective immune response against FV (Dittmer & Hasenkrug, 2000 ; Hasenkrug & Dittmer, 2000 ), we analysed the role of IL-5 and IL-6 in immunity to FV-induced disease. IL-5 is a B-cell growth factor and an IgA-enhancing factor involved in antibody responses against pathogens (Kinashi et al., 1986 ; Yokota et al., 1987 ). IL-6 is a multifunctional cytokine that plays a major role in inflammatory responses and in the maturation of B lymphocytes into antibody-producing plasma cells (Kishimoto, 1989 ). In addition, we also studied the cytokine IL-10, which has been reported to suppress the development of Th1 responses (Fiorentino et al., 1991 ). However, IL-10 has also been described as a stimulating factor for CD8+ cytotoxic T-lymphocytes (CTL) (Santin et al., 2000 ; Chen & Zlotnik, 1991 ). Since both Th1 and CTL responses have been associated with recovery from FV (Peterson et al., 2000 ; Robertson et al., 1992 ) we wanted to determine whether deficiency of IL-10 would have a positive or negative effect on recovery.

FV is a complex comprised of a replication-competent helper virus known as Friend murine leukaemia virus (F-MuLV), and a replication-defective but pathogenic virus, spleen focus forming virus (SFFV) (Kabat, 1989 ). Infection of adult mice with FV induces acute viraemia and splenomegaly of various degrees depending on the genetic background of the mouse strain (Chesebro et al., 1990 ; Hasenkrug, 1999 ). In susceptible strains, disease progresses to lethal erythroleukaemia (Moreau-Gachelin et al., 1988 ; Munroe et al., 1990 ; Wendling & Tambourin, 1978 ). Both virus-specific cellular and humoral immune responses are essential for recovery from primary FV infection (Hasenkrug et al., 1995 ; Hasenkrug & Chesebro, 1997 ; Robertson et al., 1992 ; Super et al., 1998 ). Infection of mice with F-MuLV helper virus alone protects mice from subsequent challenge with FV complex because the helper virus replicates poorly in the absence of SFFV, contains important immunological determinants and thus acts as a live attenuated vaccine virus (Dittmer et al., 1998 ). Previous experiments showed that protection with this live attenuated virus requires complex immune responses including CD4+ T cells (Th); CD8+ T cells (CTL) and B cells (antibody-producing cells) (Dittmer et al., 1999b ).

In the current study, we analysed the role of IL-5, IL-6 and IL-10 in immunity to FV infection using mice with genetic inactivations in each of these cytokine genes. Experiments using mice with deficiencies in a specific cytokine have proven to be useful models for obtaining information about the regulation of immune cells in response to infection. All mice used for our experiments were on the C57BL/6 (B6) genetic background because of the availability of cytokine genetic inactivations in this mouse strain. B6 mice are genetically resistant to FV-induced erythroleukaemia due to the Fv2 gene, which acts in a non-immunological manner to limit FV-induced polyclonal cell activation and splenomegaly (Hoatlin & Kabat, 1995 ; Persons et al., 1999 ). Despite the genetic resistance of normal B6 mice to FV-induced disease, they are not able to completely eradicate Friend virus, and infection causes a low level, life-long, persistent infection. Furthermore, experiments have shown that B6 mice deficient in specific lymphocyte subsets, such as CD4+ or CD8+ T cells, develop late-onset lethal erythroleukaemia (Hasenkrug, 1999 ). Thus, part of the resistance of B6 mice to FV-induced erythroleukaemia is mediated by specific lymphocytes, and this resistance likely involves the production of cytokines. We previously showed that it was possible to prevent the establishment of persistent FV infections as well as prevent acute disease by vaccinating mice with a live attenuated Friend helper virus (Dittmer et al., 1999a ). Such vaccine-induced protection from persistent infections was shown to be associated with complete clearance of infectious centres from the spleen by 2 weeks post-challenge. Therefore, in the current studies we analysed the role of IL-5, IL-6 and IL-10 in vaccine-induced clearance of spleen virus by 2 weeks post challenge, as well as the role of these cytokines in the resolution of acute infection.

To determine whether the B-cell stimulating cytokines IL-5 and IL-6 were important in primary FV-specific immune responses, virus loads in the plasma and spleens (for methods see Hasenkrug et al., 1998a , b ) of FV-infected B6 wild-type mice were compared with those of cytokine-deficient mice. At 1 week post-infection, all wild-type B6 mice were viraemic, with levels between 103 to 104 focus forming units (f.f.u.) per ml of plasma (mean=7762 f.f.u., SD=669 f.f.u., n=8). Both IL-5- (mean=10232 f.f.u., SD=1279 f.f.u., n=13) and IL-6-deficient mice (mean=2692 f.f.u., SD=277 f.f.u., n=14) had levels of viraemia in the same range as wild-type mice. In contrast, at 2 weeks post-FV infection, the spleen virus loads were significantly higher in IL-6-/- mice than in B6 controls (Fig. 1A). Thus, IL-6 appeared to have an important function in restricting early FV replication. In contrast, the comparison of IL-5-/- with wild-type B6 mice did not reveal any significant differences in cell-associated virus loads (Fig. 1A). In addition, we also analysed the influence of the immunomodulating cytokine IL-10 on virus loads of FV-infected mice. Deficiency in IL-10 did not have a statistically significant influence on viraemia levels at 1 week post-infection (mean=12022 f.f.u., SD=1123 f.f.u., n=16). However, at 2 weeks post-infection the cell-associated virus loads in the spleens were about 10 times higher in IL-10-/- mice than in wild-type animals (Fig. 1A). Thus, IL-10 deficiency led to impaired virus control rather then enhanced antiviral immune responses.



View larger version (10K):
[in this window]
[in a new window]
 
Fig. 1. Infectious centre assays of virus-producing cells in the spleens of mice acutely or persistently infected with FV. (A) Each dot represents the cell-associated virus load for a single mouse infected 2 weeks earlier with FV. The difference in geometric means (log10) between the B6 control group and the other three groups was analysed by one-way analysis of variance. Since the control B6 group was used for multiple comparisons, the P values were corrected using Dunnett’s multiple comparisons test. The difference between wild-type B6 mice and IL-6-/- mice was statistically significant (P<0·01), as was the difference between B6 and IL-10-/- mice (P<0·01). The limit of detection for this assay was one F-MuLV infectious centre per 3x107 spleen cells. (B) Each dot represents the cell-associated virus load for a single mouse infected 12 weeks earlier with FV. Differences between the groups of mice were not statistically significant.

 
To determine whether any of the cytokine-deficient mice progressed to leukaemia after FV infection, the mice were individually palpated for splenomegaly every week for 12 weeks following infection (for methods see Hasenkrug et al., 1998a , b ). At the end of this time-period levels of spleen infectious centres were analysed to determine whether the mice had maintained immunological control over spleen virus. None of the B6 wild-type mice nor the IL-5, IL-6 or IL-10 deficient mice became grossly splenomegalic during the observation period (data not shown). However, all of the mice still harboured persistent virus at the 3 months time-point with approximately 104 infectious centres per spleen (Fig. 1B). The level of spleen infection at the 3 months time-point was independent of the genotype of the mice, which does not point to an important role of IL-5, IL-6 or IL-10 during persistent retroviral infection.

In previous experiments we used N-tropic F-MuLV helper virus as a vaccine virus to prevent acute viraemia (Dittmer et al., 1998 ) and persistent FV infection (Dittmer et al., 1999a ). To determine the effect of IL-5, IL-6, and IL-10 deficiencies on vaccine protection from acute virus infection, vaccinated and challenged mice were assayed for spleen infectious centres at 2 weeks post challenge. Most vaccinated B6 mice were protected from spleen infectious centres after FV inoculation (Fig. 2). Results from the vaccinated cytokine-deficient mice were quite similar to wild-type mice with very low or no spleen infectious centres (Fig. 2). Thus, IL-5, IL-6 and IL-10 did not appear essential for the vaccine-induced protection elicited by infection with live attenuated virus.



View larger version (12K):
[in this window]
[in a new window]
 
Fig. 2. Spleen infectious centres in vaccinated mice at 2 weeks post-challenge with FV. Each dot represents the cell-associated virus load for a single mouse. The limit of detection for this assay was one infectious centre per 3x107 cells.

 
In order to address the role of IL-5, IL-6 and IL-10 in antibody responses of vaccinated mice we compared the antibody titres in the cytokine-deficient mice with those in wild-type animals (for methods see Miyazawa et al., 1992 ). Total virus-neutralizing antibody titres and the ability to switch from IgM to IgG were determined in these experiments. At 2 weeks post-challenge, the mean titre of total virus-neutralizing antibodies was lower in the groups of IL5- and IL6-deficient mice compared to wild-type animals (Fig. 3A). However, only the difference between IL-5-/- and wild-type B6 mice was statistically significant. The reduced levels of total antibodies in IL-5-/- and IL-6-/- mice were due to significantly lower titres of FV-neutralizing IgG antibodies (Fig. 3B), indicating an impaired class switch in IL-5- and IL-6-deficient animals. However, the reduction of humoral immune responses in IL-5-/- and IL-6-/- mice was not critical for vaccine-induced protection since both strains were well protected from FV challenge.



View larger version (11K):
[in this window]
[in a new window]
 
Fig. 3. Virus-neutralizing antibodies in mice vaccinated with F-MuLV and subsequently challenged with FV. Virus-neutralizing antibody titres from wild-type B6 mice were compared with those from IL-5-/-, IL-6-/- and IL-10-/- mice. Plasma samples were taken at 2 weeks post-FV challenge of F-MuLV-vaccinated mice. Titres of total FV-neutralizing antibodies (A) and FV-neutralizing IgG class antibodies (B) are shown. The neutralizing antibody titre was considered to be the highest dilution at which more than 75% of the input virus was neutralized. The difference between the log2 geometric mean total Ig titre of the vaccinated B6 control group was compared to the three cytokine-deficient groups by one-way analysis of variance using Dunnett’s multiple comparisons correction for comparing a control group to several experimental groups. Only the difference between B6 and IL-5-/- was statistically significant (P<0·01). The same statistical analysis was done for the IgG virus-neutralizing antibody titres. The difference between B6 and IL-5-/- was statistically significant (P<0·01), as was the difference between B6 and IL-6-/- mice (P<0·01).

 
Of the three cytokines analysed in the current study, IL-6 and IL-10 were important for controlling FV replication during acute infection whereas IL-5 played no apparent role. However, none of the cytokines affected levels of persistent virus in the mice, nor did they affect protection by vaccination with a live attenuated virus. Our results for the IL-5-/- mice are consistent with previous studies in IL-5-deficient mice showing normal B- and T-cell responses to acute influenza and vaccinia virus infections (Kopf et al., 1996 ). Thus, IL-5 does not seem to play an apparent role in the host defence against many viruses, including retroviruses. However, it remains possible that IL-5 is an important cytokine in other viral systems in which IL-5 has not been studied yet.

In contrast to IL-5, IL-6 has been reported to play a significant role in immunity to several virus infections. Our findings that secondary antibody responses against FV were impaired in IL-6-/- mice are similar to previous reports showing reduced humoral immune responses to vesicular stomatitis virus, vaccinia virus (VV) and influenza virus in IL-6-deficient mice (Kopf et al., 1994 ; Ramsay et al., 1994 ). The cellular immunity of IL-6-/- mice was found to be reduced after VV but not after lymphocytic choriomeningitis virus infection (Kopf et al., 1994 ). The impaired antiviral immune responses led to a severely compromised ability of IL-6-/- mice to clear acute infections with VV, ectromelia virus or murine cytomegalovirus (Ramshaw et al., 1997 ). Interestingly, although IL-6-/- mice were also more susceptible to herpes simplex virus type 1-induced disease (LeBlanc et al., 1999 ) and showed significantly enhanced virus replication after acute FV infection (Fig. 1), the control of persistent virus was not affected in either model. Thus, the proinflammatory cytokine IL-6 has important immune regulatory functions during acute viral infections but does not seem to play a significant role in the immunological control of persistent viruses. In all studies with IL-6-deficient mice it has to be taken into account that the lack of IL-6 affects lymphoid proliferation in vivo leading to a 20–40% reduction of T-cell numbers in comparison with wild-type mice (Kopf et al., 1994 ). Thus, some of the impairments in viral defence mechanisms of IL-6-/- mice might in part be due to lower T-cell numbers.

IL-10 is widely known as an immunosuppressive cytokine by virtue of its ability to inhibit macrophage and T-cell functions (Moore et al., 1993 ). In support of these findings, IL-10-/- mice often develop a chronic bowel inflammation due to the lack of immunosuppressive IL-10 (Kuhn et al., 1993 ). However, our results and a study with human papillomavirus (Santin et al., 2000 ) challenge the view of IL-10 as an exclusively immunosuppressive cytokine. During acute FV infection IL-10 was obviously important for restriction of virus replication in vivo (Fig. 1). Santin et al. (2000) and others (Chen & Zlotnik, 1991 ) have described that IL-10 can act as a differentiation factor for CD8+ cytotoxic T-lymphocytes (CTL). Since CD8+ T cells play an important role in immunity of mice against FV (Robertson et al., 1992 ), reduced CTL activity in IL-10-/- mice could be a likely explanation for the enhanced FV load in the spleens of these mice. In contrast to our findings, the infection of IL-10-/- mice with mouse hepatitis virus did not lead to higher virus loads compared than wild-type mice (Lin et al., 1998 ). This indicates that the involvement of IL-10 in antiviral immune responses is not a general phenomenon, but relevant only in certain virus infections.

The current results establish an important function for IL-6 and IL-10 in restriction of an acute retroviral infection. However, both these cytokines as well as IL-5 seem to be dispensable for vaccine-induced protective immunity against FV.


   Acknowledgments
 
U.D. is supported by the Deutsche Krebsforschungszentrum Heidelberg, Nachwuchsfoerderprogramm ‘Infektiologie’. The work was supported by a grant to U.D. from the Deutsche Forschungsgemeinschaft (Di 714/3-1).


   References
Top
Abstract
Main text
References
 
Chen, W. F. & Zlotnik, A. (1991). IL-10: a novel cytotoxic T cell differentiation factor. Journal of Immunology 147, 528-534.[Abstract/Free Full Text]

Chesebro, B., Miyazawa, M. & Britt, W. J. (1990). Host genetic control of spontaneous and induced immunity to Friend murine retrovirus infection. Annual Review of Immunology 8, 477-499.[Medline]

Constant, S. L. & Bottomly, K. (1997). Induction of Th1 and Th2 CD4+ T cell responses: the alternative approaches. Annual Review of Immunology 15, 297-322.[Medline]

Dittmer, U. & Hasenkrug, K. J. (2000). Different immunological requirements for protection against acute versus persistent Friend retrovirus infections. Virology 272, 177-182.[Medline]

Dittmer, U., Brooks, D. M. & Hasenkrug, K. J. (1998). Characterization of a live-attenuated retroviral vaccine demonstrates protection via immune mechanisms. Journal of Virology 72, 6554-6558.[Abstract/Free Full Text]

Dittmer, U., Brooks, D. M. & Hasenkrug, K. J. (1999a). Protection against establishment of retroviral persistence by vaccination with a live attenuated virus. Journal of Virology 73, 3753-3757.[Abstract/Free Full Text]

Dittmer, U., Brooks, D. M. & Hasenkrug, K. J. (1999b). Requirement for multiple lymphocyte subsets in protection against retroviral infection by a live attenuated vaccine. Nature Medicine 5, 189-193.[Medline]

Dittmer, U., Peterson, K. E., Messer, R., Stromnes, I. M., Race, B. & Hasenkrug, K. J. (2001). The role of IL-4, IL-12, and IFN-{gamma} in primary and vaccine-primed immune responses to Friend retroviral infection. Journal of Virology 75, 654-660.[Abstract/Free Full Text]

Fiorentino, D. F., Zlotnik, A., Vieira, P., Mosmann, T. R., Howard, M., Moore, K. W. & O’Garra, A. (1991). IL-10 acts on the antigen-presenting cell to inhibit cytokine production by Th1 cells. Journal of Immunology 146, 3444-3451.[Abstract/Free Full Text]

Hasenkrug, K. J. (1999). Lymphocyte deficiencies increase susceptibility to Friend virus-induced erythroleukemia in Fv-2 genetically resistant mice. Journal of Virology 73, 6468-6473.[Abstract/Free Full Text]

Hasenkrug, K. J. & Chesebro, B. (1997). Immunity to retroviral infection: the Friend virus model. Proceedings of the National Academy of Sciences, USA 94, 7811-7816.[Abstract/Free Full Text]

Hasenkrug, K. J. & Dittmer, U. (2000). The role of CD4 and CD8 T cells in recovery and protection from retroviral infection: lessons from the Friend virus model. Virology 272, 244-249.[Medline]

Hasenkrug, K. J., Brooks, D. M. & Chesebro, B. (1995). Passive immunotherapy for retroviral disease: influence of major histocompatibility complex type and T-cell responsiveness. Proceedings of the National Academy of Sciences, USA 92, 10492-10495.[Abstract]

Hasenkrug, K. J., Brooks, D. M. & Dittmer, U. (1998a). Critical role for CD4+ T cells in controlling retrovirus replication and spread in persistently infected mice. Journal of Virology 72, 6559-6564.[Abstract/Free Full Text]

Hasenkrug, K. J., Brooks, D. M., Robertson, M. N., Srinivas, R. V. & Chesebro, B. (1998b). Immunoprotective determinants in Friend murine leukemia virus envelop protein. Virology 248, 66-73.[Medline]

Hoatlin, M. E. & Kabat, D. (1995). Host-range control of a retroviral disease: Friend erythroleukemia. Trends in Microbiology 3, 51-57.[Medline]

Kabat, D. (1989). Molecular biology of Friend viral erythroleukemia. Current Topics in Microbiology and Immunology 148, 1-42.[Medline]

Kinashi, T., Harada, N., Severinson, E., Tanabe, T., Sideras, P., Konishi, M., Azuma, C., Tominaga, A., Bergstedt-Lindqvist, S., Takahashi, M. and others (1986). Cloning of complementary DNA encoding T-cell replacing factor and identity with B-cell growth factor II. Nature 324, 70–73.[Medline]

Kishimoto, T. (1989). The biology of interleukin-6. Blood 74, 1-10.[Medline]

Kopf, M., Baumann, H., Freer, G., Freudenberg, M., Lamers, M., Kishimoto, T., Zinkernagel, R., Bluethmann, H. & Kohler, G. (1994). Impaired immune and acute-phase responses in interleukin-6-deficient mice. Nature 368, 339-342.[Medline]

Kopf, M., Brombacher, F., Hodkin, P. D., Ramsay, A. J., Milbourne, E. A., Dai, W. J., Ovington, K. S., Behm, C. A., Köhler, G., Young, I. G. & Matthaei, K. I. (1996). IL-5 deficient mice have a developmental defect in CD5+ B-1 cells and lack eosinophilia but have normal antibody and cytotoxic T cell responses. Immunity 4, 15-24.[Medline]

Kuhn, R., Lohler, J., Rennick, D., Rajewsky, K. & Muller, W. (1993). Interleukin-10-deficient mice develop chronic enterocolitis. Cell 75, 263-274.[Medline]

LeBlanc, R. A., Pesnicak, L., Cabral, E. S., Godleski, M. & Straus, S. E. (1999). Lack of interleukin-6 (IL-6) enhances susceptibility to infection but does not alter latency or reactivation of herpes simplex virus type 1 in IL-6 knockout mice. Journal of Virology 73, 8145-8151.[Abstract/Free Full Text]

Lin, M. T., Hinton, D. R., Parra, B., Stohlman, S. A. & van der Veen, R. C. (1998). The role of IL-10 in mouse hepatitis virus-induced demyelinating encephalomyelitis. Virology 245, 270-280.[Medline]

Miyazawa, M., Nishio, J. & Chesebro, B. (1992). Protection against Friend retrovirus-induced leukemia by recombinant vaccinia viruses expressing the gag gene. Journal of Virology 66, 4497-4507.[Abstract]

Moore, K. W., O’Garra, A., de Waal Malefyt, R., Vieira, P. & Mosmann, T. R. (1993). Interleukin-10. Annual Review of Immunology 11, 165-190.[Medline]

Moreau-Gachelin, F., Tavitian, A. & Tambourin, P. (1988). Spi-1 is a putative oncogene in virally induced murine erythroleukemia. Nature 331, 277-280.[Medline]

Munroe, D. G., Peacock, J. W. & Benchimol, S. (1990). Inactivation of the cellular p53 gene is a common feature of Friend virus-induced erythroleukemia: relationship of inactivation to dominant transforming alleles. Molecular and Cellular Biology 10, 3307-3313.[Medline]

Persons, D. A., Paulson, R. F., Loyd, M. R., Herley, M. T., Bodner, S. M., Bernstein, A., Correll, P. H. & Ney, P. A. (1999). Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nature Genetics 23, 159-165.[Medline]

Peterson, K. E., Iwashiro, M., Hasenkrug, K. J. & Chesebro, B. (2000). Major histocompatibility complex class I gene controls the generation of gamma interferon-producing CD4+ and CD8+ T cells important for recovery from Friend retrovirus-induced leukemia. Journal of Virology 74, 5363-5367.[Abstract/Free Full Text]

Ramsay, A. J., Husband, A. J., Ramshaw, I. A., Bao, S., Matthaei, K. I., Koehler, G. & Kopf, M. (1994). The role of interleukin-6 in mucosal IgA antibody responses in vivo. Science 264, 561-563.[Medline]

Ramshaw, I. A., Ramsay, A. J., Karupiah, G., Rolph, M. S., Mahalingam, S. & Ruby, J. C. (1997). Cytokines and immunity to viral infections. Immunological Review 159, 119-135.

Robertson, M. N., Spangrude, G. J., Hasenkrug, K., Perry, L., Nishio, J., Wehrly, K. & Chesebro, B. (1992). Role and specificity of T-cell subsets in spontaneous recovery from Friend virus-induced leukemia in mice. Journal of Virology 66, 3271-3277.[Abstract]

Santin, A. D., Hermonat, P. L., Ravaggi, A., Bellone, S., Pecorelli, S., Roman, J. J., Parham, G. P. & Cannon, M. J. (2000). Interleukin-10 increases Th1 cytokine production and cytotoxic potential in human papillomavirus-specific CD+ cytotoxic T lymphocytes. Journal of Virology 74, 4729-4737.[Abstract/Free Full Text]

Super, H. J., Brooks, D., Hasenkrug, K. J. & Chesebro, B. (1998). Requirement for CD4+ T cells in the Friend murine retrovirus neutralizing antibody response: evidence for functional T cells in genetic low-recovery mice. Journal of Virology 72, 9400-9403.[Abstract/Free Full Text]

Wendling, F. & Tambourin, P. E. (1978). Oncogenicity of Friend-virus-infected cells: determination of origin of spleen colonies by the H-2 antigens as genetic markers. International Journal of Cancer 22, 479-486.

Yokota, T., Coffman, R. L., Hagiwara, H., Rennick, D. M., Takebe, Y., Yokota, K., Gemmell, L., Shrader, B., Yang, G., Meyerson, P. and others (1987). Isolation and characterization of lymphokine cDNA clones encoding mouse and human IgA-enhancing factor and eosinophil colony-stimulating factor activities: relationship to interleukin 5. Proceedings of the National Academy of Sciences, USA 84, 7388–7392.[Abstract]

Received 15 September 2000; accepted 29 January 2001.