Saturday, January 3, 2015

Milk remembers: Immune factors in milk “remember” childhood environments

It is well established that with very few exceptions, human milk is the preferred first food for infants. While the benefits of breastfeeding/receiving human milk are considerable and influence the development of multiple systems in the infant, perhaps the best known benefits of human milk are its immunoprotective properties. Worldwide, breastfeeding is associated with reduced risk of infectious diseases in infants, and these protections persist even in highly hygienic conditions such as the United States (Bartick & Reinhold 2010). Many immune factors are found in human milk, including immune cells, cytokines that regulate immune responses, and secretory Immunoglobulin-A (sIgA), perhaps the most common immunoprotein in human milk. It is well established that there is considerable variation in the immune factors in milk between individual mothers and between populations. It is also known that many of the immune factors in milk are highly responsive, changing in response to active infection of either the mother or infant (blog post on this topic coming next month). 

It has been traditionally held that the differences in immune factors in milk, especially sIgA, were reflecting the pathogenicity of the environment. The higher levels of sIgA found in the milk of women in developing countries was thought to be a proximate response to pathogen exposure in the immediate environment. However, some old – and some new – research suggests that the associations may be much more interesting. What if the past environment was just as important as the current environment in influencing sIgA and other immune factors in milk?

To study this, Nathavitharahna et al., (1994) decided to compare sIgA in the milk of women from three groups (sample size): women born in and currently living in Sri Lanka (n=64), women who had immigrated to England from Sri Lanka or other nearby countries in South Asia (n=20), and women born in and currently living in England (n=75).  Pathogen exposure for these groups broke down as follows: Sri Lankan women – high early life, high present; Immigrant women – high early life, low present; and British women – low early life, low present.
Surprisingly, as shown in Figure 1, there were no differences in the total amount of sIgA in the mean amounts of sIgA for each group – and within each group, milk sIgA ranged from 0.2 g/L to 19.1g/L! 

Figure 1: Comparison of total sIgA content in the milk of the three groups of mothers.
Mothers from England actually had more sIgA than mothers from Sri Lanka! However, total milk sIgA is only half the story. The researchers went on to look at specific sIgA antibodies to Escherichia coli (E. coli). They focused on 14 strands of E. coli commonly associated with moderate to severe diarrheal illness. Figure 2 is borrowed from the paper. Mothers living in Sri Lanka had the highest amount of E. coli specific sIgA in their milk. However, E. coli specific sIgA was much higher in the milk of immigrant mothers compared to their British neighbors. 
Figure 2: The amount of sIgA specific to each form of E. coli found in human milk for the three groups. Both horizontal and vertical axes are the same for each graph - white British women have much less E. coli specific sIgA in their milk than immigrant women or Sri Lankan women.

How can we interpret these findings? Nathavitharana et al., briefly considered that the pathogens may be maintained in the community, but further study demonstrated this was unlikely. Instead, the most logical explanation is that the sIgA in milk was the product of milk immunological “memory”. The sIgA in milk is produced by a type of immune cell called a B cell. These cells migrate to the mammary gland, often from the GI tract during last gestation/the onset of milk production. B cells include a special class of B cells, called memory cells. Memory B cells maintain “memories” of prior infections, allowing for a rapid antibody-mediated immune response should re-exposure occur. Memory B cells, it seems, were recording the mother’s own exposure history, migrating to the mammary gland, and providing infants with protection against the pathogenic E. coli experienced by their mother. Already having sIgA antibodies against common – and severe - pathogens may provide infants with increased capacity to resist or limit the severity of infection by these pathogens.

Six years later, another study, using a similar study design, provided further evidence for an immunological memory in milk. Holmlund et al., (2010) looked at three groups – women born and currently living in Mali (Africa), women who had migrated from Africa to Sweden (multiple countries represented), and women born and living in Sweden and analyzed their milk for several cytokines involved in immune function and sIgA. There were roughly 30 women in group. Holmlund and colleagues found few differences in the cytokines of the milks with two exceptions – Transforming Growth Factor beta ( forms 1 & 2). Women from Mali had the highest concentrations of each, with immigrant women having intermediate levels and Swedish women the lowest levels. Here’s the cool part – TGF-B2 is part of the signally cascade for sIgA, and sure enough – there was a significant association between TGF-B2 in the milk and sIgA. However, this association was only significant in the women living in Mali – but it was really, really close in the immigrant women.  

What does this all mean? It’s evidence for a complex immunological memory in milk. While this may not be important in highly hygienic environments such as the United States, it certainly suggests that there may be adaptive features in milk that record pathogen exposures during early life and provide a “dictionary” of potential infections to the infant. These highly specific forms of sIgA antibodies in human milk may allow for a more rapid immunological response by the infant. Milk “memories” therefore, would serve to protect the infant. How long these memories are retained is another question, and it seems likely that there will be rapid drift in the types of sIgA antibodies reflecting novel exposures by the infants. 

References - links included to open access papers

Bartick M, Reinhold A. 2010. The burden of suboptimalbreastfeeding in the United States: a pediatric cost analysis. Pediatrics 125(5):e1048-56. doi: 10.1542/peds.2009-1616.

Holmlund U, Amoudruz P, Johansson MA, Haileselassie Y, Ongoiba A, Kayentao K, Traoré B, Doumbo S, Schollin J, Doumbo O, Montgomery SM, Sverremark-Ekström E. 2010. Maternal country of origin, breast milkcharacteristics and potential influences on immunity in offspring. Clin Exp Immunol. 162(3):500-9. doi: 10.1111/j.1365-2249.2010.04275.x.

Nathavitharana KA, Catty D, and McNeish AS. 1994. IgA antibodies in human milk: epidemiological markers of previous infections? Archives of Disease in Childhood 71(F192-197).