Basophils are dispensable for the establishment of protective adaptive immunity against primary and challenge infection with the intestinal helminth parasite Strongyloides ratti

Abstract

Infections with helminth parasites are controlled by a concerted action of innate and adaptive effector cells in the frame of a type 2 immune response. Basophils are innate effector cells that may also contribute to the initiation and amplification of adaptive immune responses. Here, we use constitutively basophil-deficient Mcpt8-Cre mice to analyze the impact of basophils during initiation and execution of the protective type 2 responses to both, a primary infection and a challenge infection of immune mice with the helminth parasite Strongyloides ratti. Basophil numbers expanded during parasite infection in blood and mesenteric lymph nodes. Basophil deficiency significantly elevated intestinal parasite numbers and fecal release of eggs and larvae during a primary infection. However, basophils were neither required for the initiation of a S. ratti-specific cellular and humoral type 2 immune response nor for the efficient protection against a challenge infection. Production of Th2 cytokines, IgG1 and IgE as well as mast cell activation were not reduced in basophil-deficient Mcpt8-Cre mice compared to basophil-competent Mcpt8-WT littermates. In addition, a challenge infection of immune basophil-deficient and WT mice resulted in a comparable reduction of tissue migrating larvae, parasites in the intestine and fecal release of eggs and L1 compared to mice infected for the first time. We have shown previously that S. ratti infection induced expansion of Foxp3+ regulatory T cells that interfered with efficient parasite expulsion. Here we show that depletion of regulatory T cells reduced intestinal parasite burden also in absence of basophils. Thus basophils were not targeted specifically by S. ratti-mediated immune evasive mechanisms. Our collective data rather suggests that basophils are non-redundant innate effector cells during murine Strongyloides infections that contribute to the early control of intestinal parasite burden.

Fig 1. Basophil expansion in S. ratti infected BALB/c mice.

BALB/c mice were infected with 2000 L3i S. ratti s.c. into the hind footpad. Blood and mesenteric lymph node (mLN) cells were collected from each mouse at the indicated time points after S. ratti infection and basophils were stained as lineage (CD4, CD8, CD19, CD11b), c-kit (CD127) negative and IgE and CD49b positive cells and analyzed by flow cytometry (A) Gating strategy (B) Representative dot plots of blood basophils comparing day 0 and day 14 (C) Percentage of blood basophils (D) Percentage and absolute numbers of mLN basophils. Graphs in (C) and (D) show the median with 95% CI (confident interval) of two independent experiments (n = 7–8 per time point and group). *P<0.05: the infected groups differed significantly to naïve groups (day 0) as determined by Kruskal-Wallis test corrected with Dunn’s multiple comparisons test.

Fig 2. Immune response in S. ratti-infected basophil-deficient Mcpt8-Cre mice.

Basophil-deficient BALB/c Mcpt8-Cre mice (black squares) and basophil-competent littermates BALB/c Mcpt8-WT (open squares) were infected with 2000 L3i S. ratti s.c. into the hind footpad. Cytokine production by S. ratti antigen lysate (20 μg/mL) (A) or α-CD3 (1 μg/mL) (B) stimulated mLN cells derived from naïve or from day 6 infected mice was quantified by ELISA (n = 3–9). (C) IL-2 production by mLN cells derived from naïve mice cultured in medium or in the presence of αCD3 (1 μg/mL) was compared. (D) S. ratti-specific IgM, IgG1, IgG2b titers and IgE concentration in sera taken from day 14 infected mice were measured by ELISA (n = 6–9). (E) mMCPT-1 serum concentration in infected mice was quantified by ELISA at the indicated time points (n = 7). Graphs show combined data from two independent experiments, naïve mice were only analyzed once. Each symbol represents an individual mouse and bars indicate the median (A-D) or mean (E). * P<0.05: Mcpt8-Cre and Mcpt8-WT groups differed significantly, as determined by Mann-Whitney U test (A-D) or 2 Way ANOVA with Bonferroni post test (E).

Fig 3. S. ratti challenge infection in basophil-deficient Mcpt8-Cre mice.

BALB/c Mcpt8-Cre mice (dark squares/bars) and their respective littermates BALB/c Mcpt8-WT (light squares/bars) were s.c. infected with 2000 S. ratti L3i into the hind footpad. (A) Number of migrating larvae on day 2 p.i. in head and lung (n = 7–9). (B) Number of parasitic adults in the intestine on day 6 p.i. (n = 7–10). (C) Release of S. ratti 28S RNA-coding DNA in the feces at the indicated time points, as measured by qPCR (n = 8). Graphs show combined data of two independent experiments, each symbol represents an individual mouse, lines (A,B) or bars (C) indicate the median. *P<0.05; **P<0.01; ***P<0.001: significant difference between primary infection and challenge infection, as determined by Kruskal-Wallis test with Dunn’s multiple comparisons. For the kinetic shown in (C) each time point was analyzed separately.

Fig 4. Treg depletion in basophil-deficient Mcpt8-Cre mice.

BALB/c Mcpt8-WT (open circles), BALB/c DEREG (closed circles), BALB/c Mcpt8-Cre (open squares) and BALB/c Mcpt8-Cre DEREG (closed squares) mice were treated with DT on three consecutive days starting one day before s.c infection with 2000 S. ratti L3i. (A) Depletion of Tregs was controlled by flow cytometry on day 1 p.i. by intracellular staining of Foxp3. (B) Numbers of parasitic adults in the small intestine on day 6 p.i. Graphs show combined data from two independent experiments (n = 6–9), lines indicate the median. *P<0.05; ***<0.0001: Parasite burden in DEREG positive Treg-depleted and DEREG negative undepleted groups differed significantly, as determined by Kruskal-Wallis test corrected with Dunn’s multiple comparisons.