We reported previously that atypical MBCs, as a percentage of total B cells, are expanded in children and adults exposed to intense seasonal Pf
transmission in Mali 
. The objectives of the present study were to determine if atypical MBC expansion is a finding that can be generalized to Pf
-exposed individuals in Peru, and if there is a correlation between the degree of atypical MBC expansion and Pf
transmission intensity. We observed that atypical MBCs were expanded in Pf
-infected individuals in Peru relative to Pf
-naive individuals, and that the degree of atypical MBC expansion increased with increasing Pf
transmission intensity, consistent with the hypothesis that Pf
infection drives the expansion of atypical MBCs.
Although this study further strengthens the association between Pf
infection and atypical MBCs, a causal relationship remains to be established. Many factors associated with Pf
transmission could account for the increase in atypical MBCs observed in Peru and Mali, including concurrent helminth infections, cumulative exposure to other pathogenic and non-pathogenic microbes, and malnutrition. It will be important to examine the role of these and other factors in the expansion of atypical MBCs in future studies. Although we previously reported a lack of association between concurrent intestinal helminth infection and the degree of atypical MBC expansion 
, this observation needs to be confirmed in other studies. It is unlikely that HIV drives the atypical MBC response observed in the present study since the prevalence of HIV is <1.5% in the areas of Mali 
and Peru (O. Branch, unpublished data) where this study was conducted. It also seems less likely that host genetic factors play a role in atypical MBC expansion since this study suggests that atypical MBCs appear to be a generalizable finding in Pf
-exposed individuals of different ethnic backgrounds on separate continents.
From an epidemiological standpoint, additional studies are needed to further test the hypothesis that Pf
infection drives the atypical MBC response. For example, comparing B cell subsets of genetically related individuals living under similar conditions with the exception of Pf
exposure would help isolate the effect of Pf
infection on atypical MBC expansion. Tracking B cell profiles longitudinally in individuals residing in areas where Pf
transmission is rapidly decreasing 
, or in individuals emigrating from Pf
endemic areas could also provide insights into the temporal relationship between Pf
exposure and the longevity of atypical MBCs. Additionally, B cell profiles of children in the treatment and control arms of trials of intermittent preventive treatment (IPTc) of malaria 
could be compared to determine if and how decreased Pf
exposure influences the atypical MBC response.
The specificity and function of atypical MBCs in the context of Pf
infection remains to be determined. In the case of HIV, Moir et al found that HIV-specific MBCs were enriched in ‘exhausted’ MBCs whereas the total immunoglobulin and influenza-specific responses were enriched in classical MBCs 
. Further studies are needed to determine the degree to which atypical MBCs are Pf
-specific. Obstacles in this regard are the relatively low frequency of MBCs of any given antigen specificity in the peripheral circulation and the small volumes of blood typically collected in field studies. The latter could be overcome by establishing safe and reliable leukopheresis facilities where such studies are done.
Regarding the function of atypical MBCs, Ehrhardt et al,
who first described FCRL4+
‘tissue-like’ MBCs in the lymphoid tissue of healthy individuals 
, suggested that these cells may protect against invasive pathogens possibly through their influence on other cells, either directly or indirectly through the secretion of cytokines 
. In contrast, Moir et al
. suggested that this subset of hyporesponsive, or ‘exhausted’ FCRL4+
MBCs contribute to the B cell deficiencies observed in HIV-infected individuals 
. By analogy, it is possible that atypical MBCs confer protection against malaria by contributing to the regulation of the host immune response; on the other hand, it is also conceivable that exhaustion of Pf
-specific MBCs through repeated infections contributes to the inefficient acquisition and relatively rapid loss of Pf
-specific MBCs and long-lived antibodies that we have observed in the same study population in Mali 
. Immunoglobulin class switching and somatic hypermutation occur simultaneously in germinal center reactions during the differentiation of naïve B cells into MBCs. More than half of the atypical MBCs detected in individuals in Peru and Mali expressed surface IgG, indicating that at least a portion of these cells have passed through a germinal center reaction. This is consistent with models in which atypical MBCs are either the product of a dysfunctional/aborted germinal center reaction, or arise from pre-existing classical MBCs; either scenario could lead to a diminished classical MBC response.
In summary, this study provides evidence that atypical MBC expansion is a generalizable finding in Pf-exposed populations, and that the magnitude of the atypical MBC response increases with increasing Pf transmission intensity. Further studies are needed to define the origin, function and antigen specificity of atypical MBCs in the context of malaria.