The limits of protection by "memory" T cells in Ig-/- mice persistently infected with a gamma-herpesvirus

Can CD4⁺ and CD8⁺ “memory” T cells that are generated and maintained in the context of low-level virus persistence protect, in the absence of antibody, against a repeat challenge with the same pathogen? Although immune T cells exert effective, long-term control of a persistent γ-herpesvirus (γHV68) in Ig^–/– μMT mice, subsequent exposure to a high dose of the same virus leads to further low-level replication in the lung. This lytic phase in the respiratory tract is dealt with effectively by the recall of memory T cells induced by a γHV68 recombinant (M3LacZ) that does not express the viral M3 chemokine binding protein. At least for the CD8⁺ response, greater numbers of memory T cells confer enhanced protection in the M3LacZ-immune mice. However, neither WT γHV68 nor the minimally persistent M3LacZ primes the T cell response to the extent that a WT γHV68 challenge fails to establish latency in the μMT mice. Memory CD4⁺ and CD8⁺ T cells thus act together to limit γHV68 infection but are unable to provide absolute protection against a high-dose, homologous challenge.Along-term debate in the immunology community concerns the importance of antigen persistence for maintaining T cell memory (1–3). The discussion is often confused by different interpretations of the term “memory” (4). If we look at memory simply as the capacity to maintain antigen-specific, “resting” T cell numbers indefinitely, then it seems that the continued presence of the particular MHC class I or class II glycoprotein plus peptide (epitope) is certainly not required (2, 3, 5, 6). However, if memory is used in the sense of the protective immunity that might be the focus of a candidate T cell-based vaccine, then the case that continued (or sporadic) reexposure to the inducing antigen is advantageous could well have merit (7).Acutely activated T cells deal very effectively with a homologous virus challenge (8). On the other hand, the time taken to recall “resting” memory allows an invading organism to become established (9), although the pathogen may either be cleared more rapidly or (for an agent that persists) be held to a lower “set point” (10). Although cytotoxic T cells can be induced very rapidly in vivo (11, 12), their localization to (for example) the respiratory mucosa may be delayed by the need for further activation and proliferation in the lymphoid tissue (13). Could protection be made more immediate by achieving a continuing state of enhanced lymphocyte turnover (14) and activation?The herpesviruses (HVs) provide a natural system for analyzing immunity in the context of controlled virus persistence (15, 16). Vaccination strategies with the γHVs, like Kaposi''s sarcoma virus (HHV8) and Epstein–Barr virus (EBV), can be investigated (17–22) with the murine γ-herpesvirus 68 (γHV68), a virus that has high level sequence homology with HHV8 and a pathogenesis similar to that of EBV (17–19). Respiratory exposure of C57BL/6J (B6) mice to γHV68 induces transient, lytic infection of the respiratory tract and latency in B lymphocytes and macrophages (20, 21). Virus is not normally detected by plaque assay of lung homogenates for >10–12 days after the initial challenge. Genetically disrupted μMT mice that lack both B cells and antibody (Ig^–/–) also clear γHV68 from the lung and show little evidence of latency by infectious center assay (22). However, a more sensitive limiting dilution analysis (LDA) showed that γHV68 persists in macrophages and, perhaps, in other cells from Ig^–/– mice after both i.p. and intranasal (i.n.) challenge (21, 23).The present experiments ask whether the combination of CD4⁺ and CD8⁺ T cell memory (24) in mice infected once with γHV68 (25, 26) protects against superinfection with the same virus. Antibody is, of course, likely to neutralize the majority of input virus in this circumstance (27–29). Therefore, the experiments were done with Ig^–/– μMT mice (30) that had been infected with either WT γHV68 or with a mutant virus (M3LacZ) that causes a normal, lytic infection in the lung, but a much lower level of latency in the lymphoid tissue of Ig^+/+ controls (22). It is also the case that the protective capacity of immune CD4⁺ and CD8⁺ T cells that are maintained where there is the possibility of continued, low level antigen challenge has not (to our knowledge) been analyzed previously for any virus system.