Zika virus: The path to fetal infection

The question of how viruses can enter the intrauterine compartment and infect the fetus has long been a focus of research. It is of particular urgency today as the Zika virus spreads and causes perinatal infection that threatens the developing fetus with serious adverse outcomes such microcephaly and other brain anomalies, placental insufficiency, and fetal growth restriction.

We know that viruses can take a variety of routes to the fetal compartment, but we have also learned that the placenta has a robust level of inherent resistance to viruses. This resistance likely explains why we don’t see more viral infections in pregnancy.

Recent studies performed at our institution suggest that placental trophoblasts – the placenta’s primary line of defense – have inherent resistance to viruses such as Zika. It appears, therefore, that the Zika virus invades the intrauterine cavity by crossing the trophoblasts, perhaps earlier in pregnancy and prior to the development of full trophoblast resistance, by entering through breaks in this outer layer, or by utilizing alternative pathways to access the fetal compartment.

Further study of the placenta and its various cell types and mechanisms of viral defense will be critical for designing therapeutic strategies for preventing perinatal infections.

Various routes and affinities

Viruses have long been known to affect mothers and their unborn children. The rubella virus, for instance, posed a significant threat to the fetus until a vaccine program was introduced almost 50 years ago. Cytomegalovirus (CMV), on the other hand, continues be passed from mothers to their unborn children. While not as threatening as rubella once was, it can in some cases cause severe defects.

One might expect viruses to infect the placenta and then secondarily infect the fetus. While this may indeed occur, direct placental infection is not the only route by which viruses may enter the intrauterine compartment. Some viruses may be carried by macrophages or other immune cells through the placenta and into the fetal compartment, while others colonize the uterine cavity prior to conception, ready to proliferate during pregnancy.

In still other cases, viruses may be inadvertently introduced during medical procedures such as amniocentesis or transmitted through transvaginal ascending infection, most likely after rupture of the membranes. Viruses may also be transported through infected sperm (this appears to be one of the Zika virus’s modes of transportation), and as is the case with HIV and herpes simplex viruses, transmission sometimes occurs during delivery.

When we investigate whether or not the fetus is protected against particular viruses, we must therefore think about the multifaceted mechanisms by which viruses may be transmitted. With respect to the placenta specifically, we seek to understand how viruses enter the placenta, and how the placenta resists the propagation of some viruses while allowing other viruses to gain entry to the intrauterine compartment.

An additional consideration – one that is of utmost importance in the case of Zika – is whether viruses have any special affinity for particular fetal tissues. Some viruses, like CMV, infect multiple types of fetal tissue. The Zika virus, on the other hand, appears to target neuronal tissue in the fetus. In May, investigators of two studies reported that a strain of the Zika virus efficiently infected human cortical neural progenitor cells (Cell Stem Cell. 2016 May 5;18[5]:587-90), and that Zika infection of mice early in pregnancy resulted in infection of the placenta and of the fetal brain (Cell. 2016 May 19;165[5]:1081-91).

Interestingly, other flaviviruses such as the dengue and chikungunya viruses have not been associated with microcephaly or other congenital disorders. This suggests that the Zika virus employs unique mechanisms to infect or bypass the placental barrier and, in turn, to cause neuronal-focused damage.

Placental passage

The villous trophoblasts, cells that are bathed in maternal blood, form the placenta’s first line of defense. Viruses, including the Zika virus, must cross or somehow bypass this initial barrier before crossing the placental basement membrane and endothelial cells, if they are to potentially invade the intrauterine cavity and infect the fetal brain and other tissues.

Research has demonstrated that cells of various types of tissue may express certain proteins, such as AXL, MER and TYRO3. While not yet proven, these proteins may mediate the entry of viruses such as Zika, enabling them to cross the placental trophoblast layer. These proteins are indeed expressed in trophoblasts, especially in early pregnancy, but we do not yet know if the proteins actually aid Zika’s passage through the placenta.

Another mechanism that has been postulated in the case of Zika infection is antibody-dependent enhancement, a process by which a current infection is enhanced by prior infection with another virus from the same family. Some experts believe that pre-existing immunity to the dengue virus – another member of the flavivirus family that has been endemic in Brazil – may be enhancing the spread of Zika infection as antibodies against dengue cross-react with the Zika virus.

While antibody-dependent enhancement has been shown to occur and to advance infection in various body systems, it has not been proven to affect the placenta. Until we learn more, we must simply appreciate that the presence of antibodies from another member of a family of viruses does not necessarily confer resistance. Instead, it may enable new infections to advance.

One might view pregnancy as a time of immune compromise, but we have shown in our laboratories that trophoblasts in fact have inherent resistance to a number of viruses. In a recent study, we found that trophoblasts are refractory to direct infection with the Zika virus. We isolated trophoblast cells from healthy full-term human placentas, cultured these cells for several days, and infected them with the Zika virus. We then measured viral replication and compared the infectivity of these cells with the infectivity of human brain microvascular endothelial cells – nontrophoblast cells that served as a control.

Our findings were extremely interesting to us: The trophoblast cells appeared to be significantly more resistant to the Zika virus than the nontrophoblast cells.

We learned, moreover, that this resistance was mediated by a particular interferon released by the trophoblast cells – type III interferon IFN1 – and that this type III interferon appeared to protect not only the trophoblasts but the nontrophoblast cells as well. It acted in both an autocrine and a paracrine manner to protect cells from the Zika virus. When we blocked the antiviral signaling of this interferon, resistance to the virus was attenuated.

These findings suggest that while Zika appears able to cross through the placenta and infect the fetus, the mechanism does not involve direct infection of the trophoblasts, at least in the later stages of pregnancy. The virus must either evade the type III interferon antiviral signals generated by the trophoblasts or somehow bypass these cells to cross the placenta (Cell Host Microbe. 2016 May 11;19[5]:705-12).

Interestingly, the Cell study mentioned above, in which Zika infection of mice early in pregnancy infected placental cells and the brain, also showed reduced Zika presence in the mouse mononuclear trophoblasts and syncytiotrophoblasts, in areas of the placenta analogous to the human villi.

Some experts have suggested, based the study of other viruses, that the Zika virus is better able to infect the placenta when the infection occurs early in the first trimester or the second trimester. It is indeed possible – and makes intuitive sense – that first-trimester trophoblasts confer less resistance and a lower level of protection than the mature trophoblasts we studied. At this point, however, we cannot say with certainty whether or not the placenta is more or less permissive to Zika infection at different points in pregnancy.

Interestingly, investigators who prospectively followed a small cohort of pregnant women in Brazil with suspected Zika infection identified abnormalities in fetuses of women who were infected at various points of their pregnancies, even in the third trimester. Fetuses infected in the first trimester had findings suggestive of pathologic change during embryogenesis, but central nervous system abnormalities were seen in fetuses infected as late as 27 weeks of gestation, the investigators said (N Engl J Med. 2016 Mar 4. doi: 10.1056/NEJMoa1602412).

The interferon-conferred resistance demonstrated in our recent study is one of two mechanisms we’ve identified by which placental trophoblasts orchestrate resistance to viral infection. In earlier research, we found that resistance can be conferred to nontrophoblast cells by the delivery of micro-RNAs. These micro-RNAs (C19MC miRNAs) are uniquely expressed in the placenta and packaged within trophoblast-derived nanovesicles called exosomes. The nanovesicles can latch onto other cells in the vicinity of the trophoblasts, attenuating viral replication in these recipient cells.

This earlier in-vitro study involved a panel of diverse and unrelated viruses, including coxsackievirus B3, poliovirus, vesicular stomatitis virus, and human cytomegalovirus (Proc Natl Acad Sci U S A. 2013 Jul 16;110[29]:12048-53). It did not include the Zika virus, but our ongoing preliminary research suggests that the same mechanisms might be active against Zika.

Furthering research

Research at our institution and in other laboratories has shed light on various ways in which the fetus is protected from viruses, but we must learn more in order to understand how particular viruses, such as Zika, are able to reach the fetal compartment and cause particular birth defects.

We must further investigate the role and importance of antibody-dependent enhancement, and we must continue to study the placenta and its various cell types. Continuing efforts to better elucidate the placenta’s defense mechanisms and to identify cell types that are more or less resistant to the Zika virus – and understand their differences – may lead us to potential therapeutic strategies.

Dr. Sadovsky is scientific director of the Magee-Womens Research Institute, Elsie Hilliard Hillman Chair of Women’s Health Research, and professor of ob.gyn., reproductive sciences, microbiology, and molecular genetics at the University of Pittsburgh. Dr. Coyne is associate professor of microbiology and molecular genetics, and ob.gyn. and reproductive sciences, at the University of Pittsburgh.* Their research addressed in this Master Class was supported by grants from the National Institutes of Health, State of Pennsylvania Formula Research Funds, and Burroughs Wellcome Fund.

 *Correction, 7/05/2016: An earlier version of this article misstated Dr. Coyne's academic title. 

The question of how viruses can enter the intrauterine compartment and infect the fetus has long been a focus of research. It is of particular urgency today as the Zika virus spreads and causes perinatal infection that threatens the developing fetus with serious adverse outcomes such microcephaly and other brain anomalies, placental insufficiency, and fetal growth restriction.

We know that viruses can take a variety of routes to the fetal compartment, but we have also learned that the placenta has a robust level of inherent resistance to viruses. This resistance likely explains why we don’t see more viral infections in pregnancy.

Recent studies performed at our institution suggest that placental trophoblasts – the placenta’s primary line of defense – have inherent resistance to viruses such as Zika. It appears, therefore, that the Zika virus invades the intrauterine cavity by crossing the trophoblasts, perhaps earlier in pregnancy and prior to the development of full trophoblast resistance, by entering through breaks in this outer layer, or by utilizing alternative pathways to access the fetal compartment.

Further study of the placenta and its various cell types and mechanisms of viral defense will be critical for designing therapeutic strategies for preventing perinatal infections.

Various routes and affinities

Viruses have long been known to affect mothers and their unborn children. The rubella virus, for instance, posed a significant threat to the fetus until a vaccine program was introduced almost 50 years ago. Cytomegalovirus (CMV), on the other hand, continues be passed from mothers to their unborn children. While not as threatening as rubella once was, it can in some cases cause severe defects.

One might expect viruses to infect the placenta and then secondarily infect the fetus. While this may indeed occur, direct placental infection is not the only route by which viruses may enter the intrauterine compartment. Some viruses may be carried by macrophages or other immune cells through the placenta and into the fetal compartment, while others colonize the uterine cavity prior to conception, ready to proliferate during pregnancy.

In still other cases, viruses may be inadvertently introduced during medical procedures such as amniocentesis or transmitted through transvaginal ascending infection, most likely after rupture of the membranes. Viruses may also be transported through infected sperm (this appears to be one of the Zika virus’s modes of transportation), and as is the case with HIV and herpes simplex viruses, transmission sometimes occurs during delivery.

When we investigate whether or not the fetus is protected against particular viruses, we must therefore think about the multifaceted mechanisms by which viruses may be transmitted. With respect to the placenta specifically, we seek to understand how viruses enter the placenta, and how the placenta resists the propagation of some viruses while allowing other viruses to gain entry to the intrauterine compartment.

An additional consideration – one that is of utmost importance in the case of Zika – is whether viruses have any special affinity for particular fetal tissues. Some viruses, like CMV, infect multiple types of fetal tissue. The Zika virus, on the other hand, appears to target neuronal tissue in the fetus. In May, investigators of two studies reported that a strain of the Zika virus efficiently infected human cortical neural progenitor cells (Cell Stem Cell. 2016 May 5;18[5]:587-90), and that Zika infection of mice early in pregnancy resulted in infection of the placenta and of the fetal brain (Cell. 2016 May 19;165[5]:1081-91).

Interestingly, other flaviviruses such as the dengue and chikungunya viruses have not been associated with microcephaly or other congenital disorders. This suggests that the Zika virus employs unique mechanisms to infect or bypass the placental barrier and, in turn, to cause neuronal-focused damage.

Placental passage

The villous trophoblasts, cells that are bathed in maternal blood, form the placenta’s first line of defense. Viruses, including the Zika virus, must cross or somehow bypass this initial barrier before crossing the placental basement membrane and endothelial cells, if they are to potentially invade the intrauterine cavity and infect the fetal brain and other tissues.

Research has demonstrated that cells of various types of tissue may express certain proteins, such as AXL, MER and TYRO3. While not yet proven, these proteins may mediate the entry of viruses such as Zika, enabling them to cross the placental trophoblast layer. These proteins are indeed expressed in trophoblasts, especially in early pregnancy, but we do not yet know if the proteins actually aid Zika’s passage through the placenta.

Another mechanism that has been postulated in the case of Zika infection is antibody-dependent enhancement, a process by which a current infection is enhanced by prior infection with another virus from the same family. Some experts believe that pre-existing immunity to the dengue virus – another member of the flavivirus family that has been endemic in Brazil – may be enhancing the spread of Zika infection as antibodies against dengue cross-react with the Zika virus.

While antibody-dependent enhancement has been shown to occur and to advance infection in various body systems, it has not been proven to affect the placenta. Until we learn more, we must simply appreciate that the presence of antibodies from another member of a family of viruses does not necessarily confer resistance. Instead, it may enable new infections to advance.

One might view pregnancy as a time of immune compromise, but we have shown in our laboratories that trophoblasts in fact have inherent resistance to a number of viruses. In a recent study, we found that trophoblasts are refractory to direct infection with the Zika virus. We isolated trophoblast cells from healthy full-term human placentas, cultured these cells for several days, and infected them with the Zika virus. We then measured viral replication and compared the infectivity of these cells with the infectivity of human brain microvascular endothelial cells – nontrophoblast cells that served as a control.

Our findings were extremely interesting to us: The trophoblast cells appeared to be significantly more resistant to the Zika virus than the nontrophoblast cells.

We learned, moreover, that this resistance was mediated by a particular interferon released by the trophoblast cells – type III interferon IFN1 – and that this type III interferon appeared to protect not only the trophoblasts but the nontrophoblast cells as well. It acted in both an autocrine and a paracrine manner to protect cells from the Zika virus. When we blocked the antiviral signaling of this interferon, resistance to the virus was attenuated.

These findings suggest that while Zika appears able to cross through the placenta and infect the fetus, the mechanism does not involve direct infection of the trophoblasts, at least in the later stages of pregnancy. The virus must either evade the type III interferon antiviral signals generated by the trophoblasts or somehow bypass these cells to cross the placenta (Cell Host Microbe. 2016 May 11;19[5]:705-12).

Interestingly, the Cell study mentioned above, in which Zika infection of mice early in pregnancy infected placental cells and the brain, also showed reduced Zika presence in the mouse mononuclear trophoblasts and syncytiotrophoblasts, in areas of the placenta analogous to the human villi.

Some experts have suggested, based the study of other viruses, that the Zika virus is better able to infect the placenta when the infection occurs early in the first trimester or the second trimester. It is indeed possible – and makes intuitive sense – that first-trimester trophoblasts confer less resistance and a lower level of protection than the mature trophoblasts we studied. At this point, however, we cannot say with certainty whether or not the placenta is more or less permissive to Zika infection at different points in pregnancy.

Interestingly, investigators who prospectively followed a small cohort of pregnant women in Brazil with suspected Zika infection identified abnormalities in fetuses of women who were infected at various points of their pregnancies, even in the third trimester. Fetuses infected in the first trimester had findings suggestive of pathologic change during embryogenesis, but central nervous system abnormalities were seen in fetuses infected as late as 27 weeks of gestation, the investigators said (N Engl J Med. 2016 Mar 4. doi: 10.1056/NEJMoa1602412).

The interferon-conferred resistance demonstrated in our recent study is one of two mechanisms we’ve identified by which placental trophoblasts orchestrate resistance to viral infection. In earlier research, we found that resistance can be conferred to nontrophoblast cells by the delivery of micro-RNAs. These micro-RNAs (C19MC miRNAs) are uniquely expressed in the placenta and packaged within trophoblast-derived nanovesicles called exosomes. The nanovesicles can latch onto other cells in the vicinity of the trophoblasts, attenuating viral replication in these recipient cells.

This earlier in-vitro study involved a panel of diverse and unrelated viruses, including coxsackievirus B3, poliovirus, vesicular stomatitis virus, and human cytomegalovirus (Proc Natl Acad Sci U S A. 2013 Jul 16;110[29]:12048-53). It did not include the Zika virus, but our ongoing preliminary research suggests that the same mechanisms might be active against Zika.

Furthering research

Research at our institution and in other laboratories has shed light on various ways in which the fetus is protected from viruses, but we must learn more in order to understand how particular viruses, such as Zika, are able to reach the fetal compartment and cause particular birth defects.

We must further investigate the role and importance of antibody-dependent enhancement, and we must continue to study the placenta and its various cell types. Continuing efforts to better elucidate the placenta’s defense mechanisms and to identify cell types that are more or less resistant to the Zika virus – and understand their differences – may lead us to potential therapeutic strategies.

Dr. Sadovsky is scientific director of the Magee-Womens Research Institute, Elsie Hilliard Hillman Chair of Women’s Health Research, and professor of ob.gyn., reproductive sciences, microbiology, and molecular genetics at the University of Pittsburgh. Dr. Coyne is associate professor of microbiology and molecular genetics, and ob.gyn. and reproductive sciences, at the University of Pittsburgh.* Their research addressed in this Master Class was supported by grants from the National Institutes of Health, State of Pennsylvania Formula Research Funds, and Burroughs Wellcome Fund.

 *Correction, 7/05/2016: An earlier version of this article misstated Dr. Coyne's academic title. 

The question of how viruses can enter the intrauterine compartment and infect the fetus has long been a focus of research. It is of particular urgency today as the Zika virus spreads and causes perinatal infection that threatens the developing fetus with serious adverse outcomes such microcephaly and other brain anomalies, placental insufficiency, and fetal growth restriction.

We know that viruses can take a variety of routes to the fetal compartment, but we have also learned that the placenta has a robust level of inherent resistance to viruses. This resistance likely explains why we don’t see more viral infections in pregnancy.

Recent studies performed at our institution suggest that placental trophoblasts – the placenta’s primary line of defense – have inherent resistance to viruses such as Zika. It appears, therefore, that the Zika virus invades the intrauterine cavity by crossing the trophoblasts, perhaps earlier in pregnancy and prior to the development of full trophoblast resistance, by entering through breaks in this outer layer, or by utilizing alternative pathways to access the fetal compartment.

Further study of the placenta and its various cell types and mechanisms of viral defense will be critical for designing therapeutic strategies for preventing perinatal infections.

Various routes and affinities

Viruses have long been known to affect mothers and their unborn children. The rubella virus, for instance, posed a significant threat to the fetus until a vaccine program was introduced almost 50 years ago. Cytomegalovirus (CMV), on the other hand, continues be passed from mothers to their unborn children. While not as threatening as rubella once was, it can in some cases cause severe defects.

One might expect viruses to infect the placenta and then secondarily infect the fetus. While this may indeed occur, direct placental infection is not the only route by which viruses may enter the intrauterine compartment. Some viruses may be carried by macrophages or other immune cells through the placenta and into the fetal compartment, while others colonize the uterine cavity prior to conception, ready to proliferate during pregnancy.

In still other cases, viruses may be inadvertently introduced during medical procedures such as amniocentesis or transmitted through transvaginal ascending infection, most likely after rupture of the membranes. Viruses may also be transported through infected sperm (this appears to be one of the Zika virus’s modes of transportation), and as is the case with HIV and herpes simplex viruses, transmission sometimes occurs during delivery.

When we investigate whether or not the fetus is protected against particular viruses, we must therefore think about the multifaceted mechanisms by which viruses may be transmitted. With respect to the placenta specifically, we seek to understand how viruses enter the placenta, and how the placenta resists the propagation of some viruses while allowing other viruses to gain entry to the intrauterine compartment.

An additional consideration – one that is of utmost importance in the case of Zika – is whether viruses have any special affinity for particular fetal tissues. Some viruses, like CMV, infect multiple types of fetal tissue. The Zika virus, on the other hand, appears to target neuronal tissue in the fetus. In May, investigators of two studies reported that a strain of the Zika virus efficiently infected human cortical neural progenitor cells (Cell Stem Cell. 2016 May 5;18[5]:587-90), and that Zika infection of mice early in pregnancy resulted in infection of the placenta and of the fetal brain (Cell. 2016 May 19;165[5]:1081-91).

Interestingly, other flaviviruses such as the dengue and chikungunya viruses have not been associated with microcephaly or other congenital disorders. This suggests that the Zika virus employs unique mechanisms to infect or bypass the placental barrier and, in turn, to cause neuronal-focused damage.

Placental passage

The villous trophoblasts, cells that are bathed in maternal blood, form the placenta’s first line of defense. Viruses, including the Zika virus, must cross or somehow bypass this initial barrier before crossing the placental basement membrane and endothelial cells, if they are to potentially invade the intrauterine cavity and infect the fetal brain and other tissues.

Research has demonstrated that cells of various types of tissue may express certain proteins, such as AXL, MER and TYRO3. While not yet proven, these proteins may mediate the entry of viruses such as Zika, enabling them to cross the placental trophoblast layer. These proteins are indeed expressed in trophoblasts, especially in early pregnancy, but we do not yet know if the proteins actually aid Zika’s passage through the placenta.

Another mechanism that has been postulated in the case of Zika infection is antibody-dependent enhancement, a process by which a current infection is enhanced by prior infection with another virus from the same family. Some experts believe that pre-existing immunity to the dengue virus – another member of the flavivirus family that has been endemic in Brazil – may be enhancing the spread of Zika infection as antibodies against dengue cross-react with the Zika virus.

While antibody-dependent enhancement has been shown to occur and to advance infection in various body systems, it has not been proven to affect the placenta. Until we learn more, we must simply appreciate that the presence of antibodies from another member of a family of viruses does not necessarily confer resistance. Instead, it may enable new infections to advance.

One might view pregnancy as a time of immune compromise, but we have shown in our laboratories that trophoblasts in fact have inherent resistance to a number of viruses. In a recent study, we found that trophoblasts are refractory to direct infection with the Zika virus. We isolated trophoblast cells from healthy full-term human placentas, cultured these cells for several days, and infected them with the Zika virus. We then measured viral replication and compared the infectivity of these cells with the infectivity of human brain microvascular endothelial cells – nontrophoblast cells that served as a control.

Our findings were extremely interesting to us: The trophoblast cells appeared to be significantly more resistant to the Zika virus than the nontrophoblast cells.

We learned, moreover, that this resistance was mediated by a particular interferon released by the trophoblast cells – type III interferon IFN1 – and that this type III interferon appeared to protect not only the trophoblasts but the nontrophoblast cells as well. It acted in both an autocrine and a paracrine manner to protect cells from the Zika virus. When we blocked the antiviral signaling of this interferon, resistance to the virus was attenuated.

These findings suggest that while Zika appears able to cross through the placenta and infect the fetus, the mechanism does not involve direct infection of the trophoblasts, at least in the later stages of pregnancy. The virus must either evade the type III interferon antiviral signals generated by the trophoblasts or somehow bypass these cells to cross the placenta (Cell Host Microbe. 2016 May 11;19[5]:705-12).

Interestingly, the Cell study mentioned above, in which Zika infection of mice early in pregnancy infected placental cells and the brain, also showed reduced Zika presence in the mouse mononuclear trophoblasts and syncytiotrophoblasts, in areas of the placenta analogous to the human villi.

Some experts have suggested, based the study of other viruses, that the Zika virus is better able to infect the placenta when the infection occurs early in the first trimester or the second trimester. It is indeed possible – and makes intuitive sense – that first-trimester trophoblasts confer less resistance and a lower level of protection than the mature trophoblasts we studied. At this point, however, we cannot say with certainty whether or not the placenta is more or less permissive to Zika infection at different points in pregnancy.

Interestingly, investigators who prospectively followed a small cohort of pregnant women in Brazil with suspected Zika infection identified abnormalities in fetuses of women who were infected at various points of their pregnancies, even in the third trimester. Fetuses infected in the first trimester had findings suggestive of pathologic change during embryogenesis, but central nervous system abnormalities were seen in fetuses infected as late as 27 weeks of gestation, the investigators said (N Engl J Med. 2016 Mar 4. doi: 10.1056/NEJMoa1602412).

The interferon-conferred resistance demonstrated in our recent study is one of two mechanisms we’ve identified by which placental trophoblasts orchestrate resistance to viral infection. In earlier research, we found that resistance can be conferred to nontrophoblast cells by the delivery of micro-RNAs. These micro-RNAs (C19MC miRNAs) are uniquely expressed in the placenta and packaged within trophoblast-derived nanovesicles called exosomes. The nanovesicles can latch onto other cells in the vicinity of the trophoblasts, attenuating viral replication in these recipient cells.

This earlier in-vitro study involved a panel of diverse and unrelated viruses, including coxsackievirus B3, poliovirus, vesicular stomatitis virus, and human cytomegalovirus (Proc Natl Acad Sci U S A. 2013 Jul 16;110[29]:12048-53). It did not include the Zika virus, but our ongoing preliminary research suggests that the same mechanisms might be active against Zika.

Furthering research

Research at our institution and in other laboratories has shed light on various ways in which the fetus is protected from viruses, but we must learn more in order to understand how particular viruses, such as Zika, are able to reach the fetal compartment and cause particular birth defects.

We must further investigate the role and importance of antibody-dependent enhancement, and we must continue to study the placenta and its various cell types. Continuing efforts to better elucidate the placenta’s defense mechanisms and to identify cell types that are more or less resistant to the Zika virus – and understand their differences – may lead us to potential therapeutic strategies.

Dr. Sadovsky is scientific director of the Magee-Womens Research Institute, Elsie Hilliard Hillman Chair of Women’s Health Research, and professor of ob.gyn., reproductive sciences, microbiology, and molecular genetics at the University of Pittsburgh. Dr. Coyne is associate professor of microbiology and molecular genetics, and ob.gyn. and reproductive sciences, at the University of Pittsburgh.* Their research addressed in this Master Class was supported by grants from the National Institutes of Health, State of Pennsylvania Formula Research Funds, and Burroughs Wellcome Fund.

 *Correction, 7/05/2016: An earlier version of this article misstated Dr. Coyne's academic title.