Biomedical subjects
Publications and source records attributed to M K Slifka.
Despite extensive documentation of prolonged antibody responses following vaccination or acute viral infection, the mechanisms behind long-term antibody production are not fully understood. We propose the hypothesis that long-lived plasma cells are an important, yet largely overlooked, component of long-term humoral immunity.
CD40 ligand is expressed on activated T cells and interacts with CD40 on B cells and monocytes. It is not known what role CD40 ligand plays in the generation of immune responses to viral infection. To address this issue, we examined virus-specific T- and B-cell responses in CD40 ligand-deficient (CD40L-/-) mice following infection with lymphocytic choriomeningitis virus (LCMV). We found that primary anti-LCMV specific antibody responses were severely impaired in CD40L-/- mice, with the defect being most striking for antibody of the immunoglobulin G1 (IgG1) isotype. Interestingly, low levels of LCMV-specific antibodies of the IgG2a, IgG2b, and IgG3 isotypes were made in the CD40L-/- mice, showing that IgG1 responses are totally dependent on CD40L but that at least some IgG2a, IgG2b, and IgG3 responses can be CD40L independent. However, unlike CD40L+/+ mice, CD40L-/- mice were unable to sustain virus-specific antibody responses and showed a gradual decline in serum antibody levels over time. The CD40L-/- mice were also deficient in the generation of memory B cells. In contrast to the severely impaired humoral responses, CD40L-/- mice generated potent virus-specific CD8+ cytotoxic T-lymphocyte responses after LCMV infection and were able to clear the virus. These results show that CD40L does not play a role in generating primary virus-specific CD8+ cytotoxic T-lymphocyte responses but does affect the primary antibody response and the generation of memory B cells.
Listeria monocytogenes is a facultative intracellular bacterium that is able to escape phagocytic vesicles and replicate in the cytoplasm of infected cells. As with viral vectors, this intracytoplasmic life cycle provides a means for introducing foreign proteins into the major histocompatibility complex class I pathway of antigen presentation. Using recombinant L. monocytogenes (rLM) strains expressing the full-length nucleoprotein (NP) or a single cytotoxic T-lymphocyte (CTL) epitope from lymphocytic choriomeningitis virus (LCMV), we analyzed antiviral CTL responses induced by rLM vaccination. After vaccination, rLM was cleared from the host within 7 days while inducing an LCMV-specific ex vivo CD8+ effector CTL response. Virus-specific CTL memory was maintained for 6 months postvaccination, as demonstrated by vigorous secondary CTL responses after in vitro stimulation. A single immunization with rLM that expressed either the full-length NP gene or the CTL epitope alone resulted in LCMV NP-specific CTL precursor frequencies of approximately 1/10(4) CD8+ T cells. A second rLM vaccination resulted in enhanced virus-specific CTL activity and in vitro proliferation. rLM-vaccinated mice were protected against chronic viral infection by an accelerated virus-specific memory CTL response. These mice cleared infectious virus as well as viral antigen, suggesting that sterilizing immunity was achieved. In contrast to mice that received wild-type LM, rLM-vaccinated mice were protected from virally induced immunosuppression and splenic atrophy associated with chronic LCMV infection. The ability to elicit long-term cell-mediated immunity is fundamental in designing vaccines against intracellular pathogens, and these results demonstrate the efficacy of recombinant LM vaccination for inducing protective antiviral CTL memory.
Listeria monocytogenes (LM) is a Gram-positive bacterium that is able to enter host cells, escape from the endocytic vesicle, multiply within the cytoplasm, and spread directly from cell to cell without encountering the extracellular milieu. The ability of LM to gain access to the host cell cytosol allows proteins secreted by the bacterium to efficiently enter the pathway for major histocompatibility complex class I antigen processing and presentation. We have established a genetic system for expression and secretion of foreign antigens by recombinant strains, based on stable site-specific integration of expression cassettes into the LM genome. The ability of LM recombinants to induce protective immunity against a heterologous pathogen was demonstrated with lymphocytic choriomeningitis virus (LCMV). LM strains expressing the entire LCMV nucleoprotein or an H-2Ld-restricted nucleoprotein epitope (aa 118-126) were constructed. Immunization of mice with LM vaccine strains conferred protection against challenge with virulent strains of LCMV that otherwise establish chronic infection in naive adult mice. In vivo depletion of CD8+ T cells from vaccinated mice abrogated their ability to clear viral infection, showing that protective anti-viral immunity was due to CD8+ T cells.
Antiviral antibody production is often sustained for long periods after resolution of an acute viral infection. Despite extensive documentation of this phenomenon, the mechanisms involved in maintaining long-term antibody production remain poorly defined. As a first step towards understanding the nature of long-term humoral immunity, we examined the anatomical location of antibody-producing cells during acute viral infection. Using the lymphocytic choriomeningitis virus (LCMV) model, we found that after resolution of the acute infection, when antiviral plasma cells in the spleen decline, a population of virus-specific plasma cells appears in the bone marrow and constitutes the major source of long-term antibody production. Following infection of adult mice, LCMV-specific antibody-secreting cells (ASC) peaked in the spleen at 8 days postinfection but were undetectable in the bone marrow at that time. The infection was essentially cleared by 15 days, and the ASC numbers in the spleen rapidly declined while an increasing population of LCMV-specific ASC began to appear in the bone marrow. Compared with the peak response at 8 days postinfection, time points from 30 days to more than 1 year later demonstrated greater-than-10-fold reductions in splenic ASC. In contrast, LCMV-specific plasma cell numbers in the bone marrow remained high and correlated with the high levels of antiviral serum antibody. The presence of LCMV-specific plasma cells in the bone marrow was not due to persistent infection at this site, since the virus was cleared from both the spleen and bone marrow with similar kinetics as determined by infectivity and PCR assays. The immunoglobulin G subclass profile of antibody-secreting cells derived from bone marrow and the spleen correlated with the immunoglobulin G subclass distribution of LCMV-specific antibody in the serum. Upon rechallenge with LCMV, the spleen exhibited a substantial increase in virus-specific plasma cell numbers during the early phase of the secondary response, followed by an equally sharp decline. Bone marrow ASC populations and LCMV-specific antibody levels in the serum did not change during the early phase of the reinfection, but both increased about two-fold by 15 days postchallenge. After both primary and secondary viral infections, LCMV-specific plasma cells were maintained in the bone marrow, showing that the bone marrow is a major site of long-term antibody production after acute viral infection. These results documenting long-term persistence of plasma cells in the bone marrow suggest a reexamination of our current notions regarding the half-life of plasma cells.