Proportion of B cells and B cell subsets in 2008 and 2009.

<p>The mean percentages of total (CD19+) B cells (A), classical (CD19+IgD-CD27+) MBCs (B), non-class switched (CD19+IgD+CD27+) MBCs (C), naïve (CD19+IgD+CD27-) B cells (D) and double-negative (CD19+IgD-CD27-) MBCs (E) over the study period.</p

Prevalence of toxoid (TT)-specific immunoglobulin G (IgG) antibodies in highland Kenyan adults in 2008 and 2009.

<p>McNemar’s test, <i>P</i> = 1.0.</p

Number and percentages of responders to tetanus toxoid (TT) as assessed by specific memory B cells and immunoglobulin G (IgG) antibodies (Ab) in April 2009.

<p>McNemar’s test, <i>P = </i>0.71.</p

Comparison between estimated memory B cell frequencies and antibody responses to merozoite surface protein-1₄₂ (MSP-1₄₂) and tetanus toxoid (TT).

<p>There was no association between specific MBCs and antibodies for both MSP-1₄₂ and TT in 2008 and 2009.</p

Changes in B Cell Populations and Merozoite Surface Protein-1-Specific Memory B Cell Responses after Prolonged Absence of Detectable <i>P. falciparum</i> Infection

<div><p>Clinical immunity to malaria declines in the absence of repeated parasite exposure. However, little is known about how B cell populations and antigen-specific memory B cells change in the absence of <i>P. falciparum</i> infection. A successful indoor residual insecticide spraying campaign in a highland area of western Kenya, led to an absence of blood-stage <i>P. falciparum</i> infection between March 2007 and April 2008. We assessed memory B cell responses in 45 adults at the beginning (April 2008) and end (April 2009) of a subsequent 12-month period during which none of the adults had evidence of asymptomatic parasitemia or clinical disease. Antibodies and memory B cells to the 42-kDa portion of the merozoite surface protein-1 (MSP-1₄₂) were measured using ELISA and ELISPOT assays, respectively. B cell populations were characterized by flow cytometry. From 2008 to 2009, the prevalence of MSP-1₄₂-specific memory B cells (45% vs. 55%, respectively, <i>P</i> = 0.32) or antibodies (91% vs. 82%, respectively, <i>P</i> = 0.32) did not differ significantly, although specific individuals did change from positive to negative and vice versa, particularly for memory B cells, suggesting possible low-level undetected parasitemia may have occurred in some individuals. The magnitude of MSP-1₄₂-specific memory B cells and levels of antibodies to MSP-1₄₂ also did not differ from 2008 to 2009 (<i>P</i>>0.10 for both). However, from 2008 to 2009 the proportions of both class-switched atypical (CD19+IgD-CD27-CD21-IgM-) and class-switched activated (CD19+IgD-CD27+CD21-IgM-) memory B cells decreased (both <i>P</i><0.001). In contrast, class-switched resting classical memory B cells (CD19+IgD-CD27+CD21+IgM-) increased (<i>P</i><0.001). In this area of seasonal malaria transmission, a one- year absence of detectable <i>P. falciparum</i> infection was not associated with changes in the prevalence or level of MSP-1₄₂ specific memory B cells, but was associated with major changes in overall memory B cell subsets.</p></div

The proportions of marginal zone (CD19+IgD+CD27+IgM+) B cells increased between 2008 and 2009.

<p>The proportions of marginal zone (CD19+IgD+CD27+IgM+) B cells increased between 2008 and 2009.</p

Prevalence of Immunoglobulin G (IgG) antibodies to the merozoite-surface protein-1 (42) (MSP-1₄₂) in highland Kenyan adults in 2008 and 2009.

<p>McNemar’s test, <i>P</i> = 0.32.</p

Prevalence of memory B cells (MBC) to merozoite surface protein-1 (42) (MSP-1₄₂) in highland Kenyan adults in 2008 and 2009.

<p>McNemar’s test, <i>P</i> = 0.32.</p

The distribution of class-switched memory B cell subsets in adults from highland Kenya.

<p>Mean percentages of: (A) class-switched resting classical (CD19+IgD-CD27+CD21+IgM-) MBCs, (B) class-switched activated classical (CD19+IgD-CD27+CD21-IgM-) MBCs, (C) class-switched CD19+IgD-CD27-CD21+ and (D) class-switched atypical (CD19+IgD-CD27-CD21-IgM-) MBCs.</p

The magnitude of memory B cell (MBC) responses to the merozoite surface protein-1₄₂ (MSP1₄₂) and tetanus toxoid (TT) in 2008 and 2009.

<p>Magnitude of antigen specific MBCs was expressed as a percentage, calculated by diving the number of Ag-specific MBCs per million PBMCs by number of IgG-secreting cells per million PBMCs multiplied by 100. The magnitude of the MSP-1_42-specific MBC (A) and TT-specific MBCs (B) increased significantly between 2008 and 2009.</p