Assessment of the duration of protection in Campylobacter jejuni experimental infection in humans

Abstract

A human Campylobacter jejuni infection model provided controlled exposure to assess vaccine efficacy and investigate protective immunity for this important diarrheal pathogen. A well-characterized outbreak strain, C. jejuni 81-176, was investigated using a volunteer experimental infection model to evaluate the dose range and duration of protection. Healthy Campylobacter-seronegative adults received C. jejuni strain 81-176 via oral inoculation of 10(5), 10(7), or 10(9) CFU (5 adults/dose), which was followed by clinical and immunological monitoring. Based on dose range clinical outcomes, the 10(9)-CFU dose (n = 31) was used to assess homologous protection at 28 to 49 days (short-term veterans [STV]; n = 8) or 1 year (long-term veterans [LTV]; n = 7) after primary infection. An illness dose effect was observed for naïve subjects (with lower doses, 40 to 60% of the subjects were ill; with the 10(9)-CFU dose, 92% of the subjects were ill) along with complete protection for the STV group and attenuated illness for the LTV group (57%). Partial resistance to colonization was seen in STV (25% of the subjects were not infected; 3-log-lower maximum excretion level). Systemic and mucosal immune responses were robust in naïve subjects irrespective of the dose or the severity of illness. In contrast, in STV there was a lack of circulating antibody-secreting cells (ASC), reflecting the local mucosal effector responses. LTV exhibited comparable ASC responses to primary infection, and anamnestic fecal IgA responses likely contributed to self-resolving illness prior to antibiotic treatment. Campylobacter antigen-dependent production of gamma interferon by peripheral blood mononuclear cells was strongly associated with protection from illness, supporting the hypothesis that TH1 polarization has a primary role in acquired immunity to C. jejuni. This study revealed a C. jejuni dose-related increase in campylobacteriosis rates, evidence of complete short-term protection that waned with time, and immune response patterns associated with protection.

FIG. 1.

Selected clinical and microbiological outcomes of C. jejuni infection (upper panels) and times to clinical and microbiological events (lower panels). The groups studied were naïve subjects who were inoculated with 10⁵, 10⁷, and 10⁹ CFU and STV and LTV who received 10⁹ CFU. The box plots show medians, interquartile ranges, and minimum and maximum values. The first diarrheal stool was the first loose or liquid stool fulfilling the definition of diarrhea. Clearance data were based on the time of the last C. jejuni-positive stool culture (time zero was the time when the first antibiotic dose was given). The illness severity index (SI) was calculated using gastrointestinal symptoms (abdominal cramps, nausea, vomiting, tenesmus, and dysentery), systemic symptoms (chills and sweats, malaise, joint aches, loss of appetite, and headache), the maximum temperature, and diarrhea (highest value for stool frequency or volume over the illness episode). The composite score was calculated as follows: symptom score (0, no symptoms; 1, grade 1 symptoms; 2, grade 2 symptoms; 3, 1 or 2 grade 3 symptoms; 4, ≥3 grade 3 symptoms) plus the maximum temperature score (0, <38.1°C; 1, 38.1 to 38.3°C; 2, 38.4 to 38.6°C; 3, 38.7 to 38.8°C; 4, >38.8°C) plus the diarrhea score (0, no loose or liquid stools; 1, <3 stools or <500 ml; 2, 3 to 6 stools or 500 to 999 ml; 3, 7 to 9 stools or 1,000 to 1,499 ml; 4, >9 stools or >1,500 ml). The SI ranged from 0 to 16. The P values for comparisons of naïve subjects who received 10⁹ CFU and subjects who received lower doses were as follows: for median incubation time, Breslow P = 0.0198 and log rank P = 0.0038; and for median time to infection, Breslow P = 0.0004 and log rank P = 0.0003.

FIG. 2.

Serologic responses following experimental infection with C. jejuni. The groups studied were naïve subjects who received inocula containing 10⁵, 10⁷, and 10⁹ CFU and STV and LTV who received 10⁹ CFU. Data were obtained for IgG, IgA, and IgM.

FIG. 3.

IgA-specific antibody-secreting cell (ASC) responses following C. jejuni infection. The groups studied were naïve subjects who received inocula containing 10⁵, 10⁷, and 10⁹ CFU and STV and LTV who received 10⁹ CFU. The response shown are the maximal responses between 6 and 9 days postinoculation. The horizontal lines indicate medians. No ASCs were detected before challenge in any group (data not shown).

FIG. 4.

Fecal IgA responses following experimental C. jejuni infection. The groups studied were naïve subjects who received inocula containing 10⁵, 10⁷, and 10⁹ CFU and STV and LTV who received 10⁹ CFU.

FIG. 5.

In vitro IFN-γ responses following C. jejuni infection. The groups studied were naïve subjects who received inocula containing 10⁵, 10⁷, and 10⁹ CFU and STV and LTV who received 10⁹ CFU. The preinoculation values (Pre) were determined on the day of inoculation. The postinoculation values (Post) are the maximum differences between the values obtained either 28 or 60 days postinoculation and the preinoculation values.