Duration of peripheral blood cellular immune responses induced in dogs in response to a modified live attenuated vaccine for Ehrlichia chaffeensis

Abstract

Abstract Ehrlichia chaffeensis (ECH) is an obligate intracellular gram-negative bacterium and a frequent cause of severe and fatal tick-borne infection in people in North America. The canine is a common incidental host and a useful model for vaccine development and assessing immune responses to ECH infection. We recently established mutagenesis methods for ECH and demonstrated that immunization of dogs with an ECH mutant with a functional disruption in the ECH_0660 gene protected the vertebrate host from both tick-transmitted and intravenous wildtype ECH challenge when given 4 weeks after immunization. In the current study, we evaluated the duration of immunity offered by the ECH_0660 gene mutant live attenuated vaccine (MLAV) against wildtype infection challenge. Groups of 12 dogs were immunized with the ECH MLAV and then challenged 2, 4, 8 and 12 months later via tick-transmitted or blood-transmitted infection with wildtype ECH. Animals were monitored for 4 weeks after challenge and then sacrificed for evaluation of tissue pathology and bacterial burden. Immunization with the MLAV induced ECH-specific T cell responses, detectable by ELISPOT for IFNγ; and differentiation of IFNγ- and/or TNFα-producing ECH-specific T effector memory CD4 T cells (CD45RAneg CD45RO+ CD62Lneg). Antigen-specific recall responses were detectable in the peripheral blood of most (27/32) vaccinated dogs for 4 months after immunization, but only in 5/24 of the dogs after 8 months. This result suggests that a single MLAV vaccination induces cellular immune responses and differentiation of memory helper T cells. Future work will investigate the correlations between cellular immunity, humoral responses and bacterial load. Supported by grants from NIH (R01 AI152418)